The USSR fell in the end because the country's organizational, labor allocative, motivational methods were inherently uncompetitive. If it fell in the 50 you might have had an argument, but it fell in the 80-90ies, two generations later. The population especially the intelligentsia lost any will to participate in the soviet project. Gorbachev's perestroika with its increased freedom of speech strongly demotivated everyone, because it was evident that the way things are done is just wrong, and they were very poor compared to their supposed western opponents. Even the KGB under perestroika lost its main suppressive purpose. They over-urbanized the population, didn't make rural kolkhoz life attractive enough due to their ideology of everyone gets the same shit, hence issues with agriculture and food production, resulting in dependence on foreign imports. That worsened during perestroika and further demotivated the population. The dissolution of the empire is just a consequence of the dissolution of the political regime in general.

The USSR would have been under pressure from the west anyway, regardless of several additional millions in 40ies or 90ies. Simply because the west and their allies like Japan and South Korea were many times the population size and many times the economic efficiency per capita. 5-10 additional million is a drop in a bucket against such an opponent, and all the "veterans" have reached the pensioner age by the end of the 80ies, were non-active and sucking resources, the less of them the better. The economic and motivational issues don't come from the lack of manpower like ever. If there was food on the table, and no freedom of speech there wouldn't be any "revolutionary situation".

Without perestroika it wouldn't have fallen, and perestroika wasn't inevitable. Just a fluke. Could have been another strongman instead of Gorby, who could have modernized the economy China style, or just increased repression and control (Russians are famous terpilas) Although who knows, if we assume the general cultural trends it's possible that a character like Gorby was inevitable, and with that the collapse of the USSR was also inevitable.