the USSR collapsed because of economic pressures. with the largest sponsor gone, others gave up too.
the collapse of the USSR was avoidable, and it is paramount that socialists of the 21st century understand why it collapsed.
there were political reasons too, but my opinion is that even with their political system, it could have survived if the economic system was reformed. ironically, the USSR not being socialist enough is what destroyed it: it was, at the end of its lifespan, neither good at being capitalist, nor good at being socialist. being stuck in between is what led to the stagnation. the answer would have been cybernetic planning and an information technology revolution. with the base of the economy changing, the superstructure would have changed as well. perhaps the system would be more open, the siege mentality would be lessened, the democratic inputs would have been increased and the cynicism of the 80s would have been replaced with the optimism of the 50s and 60s.