Peter Zeihan (who used to run analysis for Stratfor but is now running his own organization) has a different take on Russian motivations. I'll see if I can summarize...
When the USSR collapsed and the Warsaw nations mostly broke free, Russia lost control of the geographic features between which they could park relatively non-mobile forces in order to block access to the plains that make up a lot of Russia. Expanding outwards until borders match up with geographic boundaries has been a thing for the Muscovites since pretty much forever. The RF is in demographic collapse, so they have to grab control of those western geographic boundaries this decade or it won't happen. Ukraine is on the way to two of those boundaries, so if the RF wins here, they're not stopping there.
If Putin accidentally falls out a window or on some bullets, whoever succeeds him will almost certainly see the same issue, and launch another war - if they can.
- End attempted summary -
Personally, I come down between this and the threat to internal stability a free, democratic, and prosperous Ukraine might pose to the RF, particularly given the shared language and cross-border families - with a soupcon of threat to the RF's piggybank that petro in Ukraine might represent. I'd also agree that it's impossible to negotiate with Putin - but I'd expect similar issues with whomsoever comes to power after him. Even Navalny seems to be an imperial nationalist.
Bright side: we just need to hold them back for the next decade or so and they'll be unable to invade neighbors. Supporting Ukraine with our entire arsenal, and then rebuilding Ukraine is the best way we can do that. As it is, they have the best army in Europe, once this war is over, they'll make a spectacular new NATO member.
