I am no expert. I will though quote a Chinese comrade i spoke with about this.
"The USSR's revolution was incomplete."
This was in reference to how they had reactionaries inside the government. Contrary to how it is portrayed the USSR was lenient in a lot of ways. It gave far too much room for liberals to exist. Especially after Stalin died. Although even he was a lot nicer to them than he should have been honestly. This was done with good intentions of course. Not wanting to be too strict. It was still a mistake.
Putin was a KGB agent. Does he seem like someone who should have been trusted with a position like that? These were the cracks exploited to bring down the USSR. It was the first socialist project it didn't have hindsight like we do. It did a lot of amazing things, but it had so many enemies. The fact is survived as long and accomplished as much as it did is astounding.
In short, the USSR fell slowly, death by 1000 cuts.
But the USSR's legacy persists despite is dissolution. The spectre of communism hangs not over Europe, but over the world. The torch has been passed, and the fight continues.