Short answer to the question: This wouldn't have made sense to anyone at the time.

Long answer to the question (it's going to be very long, sorry):

The Czechoslovak government-in-exile, which included Slovaks, absolutely insisted on the pre-Munich borders. There was talk about a Slovak Soviet Socialist Republic among the Slovak Communists during the uprising in 1944, but it was basically their attempt to stake out an extreme position so that the government-in-exile and the leadership of the Czechoslovak Communist Party would take their demands for federalism more seriously, and I don't believe there's any indication the Soviets themselves were interested. The loss of Subcarpathian Ruthenia to the USSR demonstrates the Czechoslovak government's willingness to give in to Soviet demands, and probably also the extent to which its leaders cared all that much about Subcarpathian Ruthenia in the first place, not any lack of commitment to Czechoslovakia.

The Czechoslovak government was reestablished in Košice on April 5, 1945, while most of the Czech lands were still under Nazi occupation. Although this government wasn't outright dominated by the Communists, it was quite left-wing, and it was totally committed to a military alliance with the Soviet Union. This goes for the non-Communist members as well as the Communists, who believed -- for what seemed like good reason to them -- that only the USSR was both powerful enough and willing enough to defend Czechoslovakia's territorial integrity. Czechoslovak leaders made the most important decisions regarding their country's future well before any major Czech cities were liberated by anyone. I don't know enough about the military history of the Second World War to say whether the Western allies could just as easily have liberated the Czech lands as the Red Army, but I doubt it's as simple as that. Yes, the Americans could have continued on to Prague and gotten there before the Soviets did, but by that time, Brno had been liberated for a week and a half. In any case, both the US and the Soviet armies were out by the end of the year.

Czech postwar history is sometimes told like a sort of tragic ballad that goes something like this: The Czechs have every right to be considered Westerners, they are wealthy, developed, educated, and civilized, they demonstrated their commitment to democracy during the interwar period, they're a part of Western European high culture and art, and what a tragedy it is that they got locked behind the Iron Curtain with the despotic, eastern Russians! This isn't really an argument, so it can't be either proven or refuted. Quite a few historians have looked into how it has developed and what political purposes it's been put to over the decades. Historians have also investigated questions of Czechoslovak democracy and economic prosperity during the First Republic and developed a picture of the period that's much more complex and interesting than the idealized version put forth in the ballad, even though it remains absolutely true that Czechoslovakia was the only Central European country to maintain a liberal democracy between 1918 and 1938 and that its citizens enjoyed rights that people in surrounding countries did not. Western observers who subscribed to this narrative found the events in Czechoslovakia in 1948 especially disturbing: if the civilized Czechs with their democratic political culture could fall under the spell of the Communists, so could the French, or the English, or the Americans! This is why Communists have to be kept as far away from power in democratic countries at all possible cost!

There is also a narrative about the relationship between the Czechs and Slovaks that paints the Czechs as enlightened Westerners and the Slovaks as backward, benighted Eastern bumpkins. It is, of course, true that the Czech lands are located to the west (well, northwest) of Slovakia, that they have historically been wealthier and their economy has been more industrial and less agricultural, and that the political circumstances of the Kingdom of Hungary were different from those in Austria. During the early twentieth century, this narrative was used to convince Czechs that they had an obligation to help their poor little brothers and raise them to their level -- in fact, by helping the Slovaks, the Czechs could confirm that they were enlightened Westerners. It's possible that some Czechs turned it around and used it the way you suggest they might have in your question -- i.e., "What do we need these backwards Slovaks for? If we ditched them, we could join the West" but I haven't seen it, either during the interwar or the post-WWII periods. It was a major theme during the 1990s, though.

Regardless of the achievements of interwar Czechoslovakia, these narratives are unfair and disparaging towards the Slovaks and other nations that it constructs as "Eastern" (including the Germans and Austrians, sometimes), and also disregard the authoritarian politics and poverty that existed in the Czech lands as well as in indisputably Western countries during the interwar period. They're based in a view of world history that sees an essential division between the uncivilized East and the civilized West which simply doesn't hold up and doesn't make a lot of sense. It's best just to study Czechoslovakia -- or any country -- on its own terms.

What's funny about all of this is that, among all the countries in Central Europe, support for both the Soviet Union and for the Communists may have been highest among the Czechs. The Czechoslovak Communist Party got forty percent of the vote in the Czech lands in 1946, and they had about a million members out of a population of a little less than nine million (including children, who obviously could not join political parties). This isn't to say that everyone supported communism, and it's definitely not to say that very many people would have wanted the repressive, Stalinist regime that they ended up with, but discussions of postwar Czechoslovak politics don't make any sense without taking popular support for the Communists into account. In Slovakia, the Communists got a smaller percentage of the vote, although the results can't really be compared directly since different parties were competing. Why the Soviet Union and the Czechoslovak Communist Party had such popular support among the Czechs is a separate question (and a very interesting one).

So the question of whether the Western allies "let" Czechoslovakia fall to communism assumes that the Great Powers determined Czechoslovakia's postwar political development. In one sense, it's not exactly wrong: it's hard to argue Czechoslovakia wouldn't have been part of the Soviet bloc if the Czechoslovak Communists hadn't had popular support (see Hungary and Poland). At the same time, though, it's also hard to argue that the Communists wouldn't have been very influential in postwar Czechoslovakia regardless of what the Western powers or the USSR did. If what you're interested in is the geopolitics of the early Cold War, it may be okay to focus on the actions and motivations of the Great Powers. But if you're interested in Czechoslovakia, it's crucial to understand the independent decisions of Czechoslovak political leaders and the domestic context in which they were made.