I think it is accurate to refer to the existence of a conspiracy as a misleading half-truth, yes. The most important thing to know is that nobody has ever found proof that such a conspiracy existed, and us transit scholars have a wide variety of other explanations for what happened to streetcars that don't really fit in with the theory that it was all a conspiracy.

The first thing to address is, well, the trial that led to the theory that this was a conspiracy. There was a bus company called National City Lines, which ran transit systems in at least 44 US cities, and a consortium of oil, tire, and automotive companies, including General Motors, Firestone Tire, and Standard Oil of California, did invest in National City Lines. These companies did engage in monopoly practices in the sale of vehicles, fuel, and supplies and such to NCL and subsidiaries. We know this because they were tried in court, and found guilty in 1949. However, they were aquitted of the charges of monopolizing public transportation. This trial is the origin of the story of there being a conspiracy on the part of GM in particular to destroy streetcars, but there isn't any actual evidence of this to be found within the trial. A conspiracy existed, yes, and crimes were committed, but it does not follow that this is the reason for the fall of the streetcar in American cities.

The issue with blaming the fall of streetcars on the conspiracy is that by the time National City Lines went around purchasing all of these different transit systems, the transit systems were already struggling massively. That's why they were so cheap to buy up. Streetcar operators by and large were not fiscally solvent, so even if there were such a conspiracy, it would not explain the loss of streetcars in American cities, as the seeds for this loss were planted well before National City Lines came around.

Let us go back to how streetcar lines were originally constructed and paid for in US cities to understand why this is. The streetcar is actually the original driver of American suburbanization, not the automobile. In the 19th Century, in order to allow for lower density suburban developments (though nowhere near as low density as post WW2 suburbs), real estate developers used new electric traction technologies to construct streetcar lines to serve the developments they were constructing. This is fundamental to understanding the fall of the streetcar. These streetcar networks never actually made money, and that was okay. While they operated at a loss, this was much more than made up for by the massive spike in real estate values anywhere they existed. This is in fact quite similar to how railroad tycoons made their vast fortunes, and the rule that profitable passenger rail services make their money from real estate that derives value from having adjacent rail service persists today, and does a lot to explain why American public transportation that is usually either not allowed to enter the real estate business or possesses limited real estate tends to consistently be a money loser.

So, streetcars did not typically make money, even before the car came along, and cities were not usually paying for streetcar systems. Exceptions to both of these rules did exist, and there are a handful of examples of either independently solvent systems or public systems, but by and large this is how streetcars were financed and run, and by whom. This would become a problem when real estate developers, satisfied they'd made their profit, started looking to get out of the money losing streetcar business, and offloaded the systems. In most cities, this worked on a franchise system, where the right to operate on a given street was auctioned off every so many years, and with it came the responsibility to maintain the right of way. The problem inherent in this is, again, the systems never really made money, and there was no tradition of cities stepping in to fund these services at a loss. In some cities it did happen, such as in San Francisco, where the city bought up franchises as they expired to run a municipal system that still exists today, though streetcars are now a very minor part of that system, but this was the exception, not the rule. And, well, by the time 1920 rolls around and the economic crises facing streetcars were coming to a head, the car is increasingly present and affordable to more and more Americans. It is important to emphasize that streetcars were not making a profit in the vast majority of circumstances even before they began competing with cars for riders, so when this competition got particularly fierce as cars became attainable, it created a death spiral for streetcar systems.

Cars were the new, sexy technology, and Americans by and large fell in love with them, which created intense pressure on cities to construct infrastructure for the new devices. This is also important to understand why the conspiracy theory does not explain the loss of streetcars. Automobile infrastructure is just as much of a money loser as streetcars. When we factor in capital costs, cars do not make money at a societal level, it's just, the consumer pays for these costs out of nontransparent taxes and fees, so it isn't clear to most people that, while public transit is a money loser, so is the automobile. I want to emphasize this, because I do not want to come off sounding like I am saying that public transit isn't a social good. Like most urban planning students, I favor public transit infrastructure over automobile infrastructure, and as much as public transit loses money, it is not as if automobile infrastructure is not also a money sink.

So cities were stepping in to fund transportation at a loss, it is just that the public demand was for automobile infrastructure, not bailing out failing streetcars. To reiterate, the voting public was pressuring politicians to fund cars over streetcars. Blaming the fall of the streetcar on the conspiracy by GM and co fundamentally does not track, because the political winds were blowing towards the automobile well before it happened. Furthermore, the seeds of the post-WW2 suburban boom were laid in the 20s and 30s, but do to the Great Depression and WW2, this boom was placed on hold, and the fall of the streetcar was delayed. With the end of the war, however, there was a gigantic housing crisis in the US, which did not have affordable homes for returning veterans. This is how the archetypal post war suburb came to dominate the US. We needed a lot of housing, fast, and cookie cutter style single family homes on cheap land can be built quickly and relatively cheaply, especially with the federal government being willing to step in and help veterans pay for homes they wouldn't otherwise be able to afford. By this point, however, streetcar systems were already on their last leg.

What National City Lines did was, in most cities, abandon the streetcar in favor of the bus, which is the origin of the conspiracy theory that this was an intentional attempt to the destroy the streetcar. Which I guess could be said to be true insofar as NCL replaced expensive to maintain rail infrastructure with cheaper buses that could be rerouted and used to serve lower ridership areas cheaper, but does not itself indicate a conspiracy to destroy public transportation, just a decision that buses were often preferable. This is further backed up by the fact that NCL did maintain the streetcar systems in several cities, generally the densest and most profitable systems. Again, to be clear, there was a conspiracy, and there was criminal behavior. It is just not the case that this conspiracy was the cause of the death of the streetcar or that the streetcar would have lasted longer without the conspiracy, or that the conspiracy was intended specifically to destroy the streetcar. The conspiracy that can be proven was price fixing of buses and fuel and supplies related to buses.

Further reading, if you'd like to go deeper into the subject:

Kennedy, 60 Minutes, and Roger Rabbit: Understanding Conspiracy-Theory Explanations of The Decline of Urban Mass Transit

Presentation from the 78th Annual Meeting of the Transportation Research Board, Washington, D.C. January, 1999

http://marthabianco.com/kennedy_rogerrabbit.pdf

United States v. National City Lines

The decision from the lawsuit regarding the conspiracy.

https://caselaw.findlaw.com/us-supreme-court/334/573.html

"A Streetcar Named Conspiracy." The Wilson Quarterly (Washington) 22.3 (1998): 10. Web.

A journal article explaining how the conspiracy theory regarding the loss of streetcars just doesn't track with events as they occurred.

Diers, John. "Did a Conspiracy Really Kill the Streetcar?" Trains 66.1 (2006): 56. Web.

Another overview of events as they happened, focusing on the crucial story of the Twin Cities.

Diers, John W., and Aaron Isaacs. Twin Cities by Trolley: The Streetcar Era in Minneapolis and St. Paul. Minneapolis: U of Minnesota, 2007. Web.

A history of streetcars in the twin cities, written by the guy from the last source.

Akimoto, Fukuo. "California Garden Suburbs: St. Francis Wood and Palos Verdes Estates." Journal of Urban Design 12.1 (2007): 43-72. Web.

A journal article going over some of the fundamentals of streetcar suburbs.