If history repeats itself, which it often does, Putin is in big trouble. Last collapse happened after 15,000 troops died in the Afghan war. Now they've lost easily 4x that number in 1/5th the time.

Even if there's no coup/revolution/he doesn't get assassinated/etc.

He's 70, and very possibly not in the best health. Statistically it wouldn't be very surprising if he dropped dead of natural causes at any moment. Technically he's already a small anomaly of a Russian male just because he has lived as long as he has since their average life expectancy is currently something like 65 (which is admittedly skewed very young by a whole lot of stuff like drinking, drug use, suicide, and being shipped off to Ukraine) And even if he beats all odds and lives another 30 years it still shouldn't be unexpected for him to just decide to step down and retire at some point. Shit, if I had a fraction of the money and assets he has, I'd have called it quits a long time ago and disappeared from the public view.

And when he goes, no matter how he goes, there's going to be a power vacuum that needs to be filled and a whole lot of assholes jockeying for position. I won't pretend to know how that will go, maybe there will be all-out civil war, maybe just a handful of the right people will fall out of windows or drink polonium tea or just get disappeared, maybe there will be a major economic collapse and a rise in crime rates, maybe everything will just be hunky-dory and it really was just Putin holding everything back and Russia will enter a new age of enlightenment (it will probably not be that last one)