The reason Ukraine hasn't been allowed to join NATO already is because the rest of the alliance isn't willing to make that promise (that they'll go to war with Russia to protect Ukraine).
It's doubtful Russia would go to fight Ukraine even now (it would be rather disastrous), but even more doubtful if Ukraine would join NATO.
Ukraine has its own internal problems for now.
Unfortunately it's still possible that Putin will cling to power for a rather long time.
The fact that they (professionally) can't do anything else, and that a pro-democratic alternative to Putin would likely severely cut their numbers (Russia has one of the highest if not the highest number of siloviks per capita) or even conduct lustrations against them for protecting the autocracy, plays into Putin's hand. They are also not that demanding, higher ups known to "live of the land" (protection racket etc), and Russian economy even under sanctions still a capitalist economy, so it won't be as bad as in the 80ies.
Also Russia coming under isolating western sanctions would effectively make Russia a China's tributary state, since it's the only other power with manufacturing capability Russia would be able to trade with. So that adventure would be exchanging Putin's and silovik interests for Russian future BIG time.