The humvee was a replacement for the jeep, also an unarmored vehicle. The point of the humvee is mobility. You need to move soldiers, you need to move equipment (though there are usually other vehicles for that), you need to have flexibility on the battlefield. In general, the humvee was not designed to be a front-line vehicle, it was designed as a support vehicle behind the lines, just like the jeep, with only a few very specific uses in the "heat" of battle.

Humvees were used extensively in the first gulf war, for example, which was about as close as you could get to a typical Cold War era hypothetical US vs. USSR conventional engagement of land forces. But they were not used as front line units. Armored units such as tanks (such as the Abrams) and APCs (such as the Bradley IFV) were used to engage enemy armor in combination with air power. Humvees were mostly used to shuttle around soldiers behind the front lines, not in front of them.

In general, the US armed forces were designed to fight these sorts of major organized force on force engagements during the Cold War, they weren't designed with insurgency fighting in mind. And you can see this weakness in the Somali peace keeping mission of the early 1990s. You can also see it even more pronounced with the Russian military fighting in Afghanistan in the '80s and in Chechnya in the '90s. Neither the Russian nor American forces were prepared for fighting in situations where the front-line was amorphous and everywhere, where an attack could come at any time, where IEDs and mines could hit unarmored vehicles operating in "peaceful" areas.

The traditional armored/unarmored mix of vehicles is not suitable for those scenarios, because tanks and APCs are way too big to be used for everyday transportation while unarmored vehicles are too vulnerable to ambush attacks (I won't cover the development of MRAPs since that's a sub-20 year phenomenon that's out of scope on AskHistorians).

However, consider the Battle of Grozny in the First Chechen War. On new year's eve in December 1994 several columns of Russian armor (tanks and BMPs) drove into downtown Grozny where they were besieged by heavy RPG and automatic weapons fire. Over 200 Russian armored vehicles were destroyed and the Russians had to go into retreat. So even having the protection levels of a modern main battle tank is no guarantee that you won't get blown to smithereens by small arms and ambush tactics in an urban environment. Protecting against ambush attacks in counter insurgency operations is extremely difficult, so it's no wonder that it was de-prioritized by armies that didn't see counter insurgency as a major role for their forces.

In short, the humvee was designed for a different role and a different battlefield than counter insurgency campaigns.

I'll preface by saying I'm not a historian, just interested in amazing military vehicles that are honestly feats of engineering.

Not all HMMWVs are light weight, although when you consider them against the MRAP or the newer JLTV that is beginning production they are definitely a lighter weight vehicle with less protection.

The HMMWV was originally created when the US Army asked AM General, Chrysler, and Teledyne continental to create around 12 vehicles to prototype, with the expectation that a large contract would be rewarded afterwords. The army was looking for a vehicle that could be utilized in a variety of different ways with small changes, essentially looking for a common frame and powertrain, but with the ability to easily swap bodies and add extras onto the vehicle. This vehicle would be replacing the Jeep that was popularized during WWII.

In late March of 1983 the Army awarded the contract to AM General to enter production for the HMMWV design, and requested around 55,000 vehicles. The Air Force, Marine Core, and Navy would also add orders to bring the total to close to 70,000 vehicles.

The vehicles would be entered into the field for use during the invasion of Panama (1989) as well as into the gulf war in early 1990s.

The main issue with being under armored came about in 2003-2004 when the US went from fighting the Iraqi insurgency, to trying to regulate and combat guerrilla warfare. Most HMMWVS were adequate at protecting the occupants from projectiles from the side, but if a bomb was to explode beneath them, they were left nearly defenseless. The US Army requested to have more protection kits made that could help in deflecting bombs from below, such as IEDs.

However, armoring a vehicle is not easy, and as you can imagine adds a lot of weight to a vehicle that wasn't originally designed to support the extra 1,000 to 2,000 lbs of weight. Some drawbacks of the kit was increased sway due to a single rear wheel running independent suspension, as well as doors not opening after a blast.

I am not a historian however I read up on HMMVWs a few years ago. I apologize if any information is incorrect or if it appears I was speculating.

Did the brass just expect to fight the Russian horde in rag tops? What was the plan, expendable troops?

This seems to be politically charged so I will refrain from speaking my opinion on the matter.