No person seriously discussing that region of the world would have ever thought Ukraine would give Crimea to the Russians without a fight. Countries, in general, don't give up land without a fight. Crimea is also one of the most militarily valuable pieces of land in the world. Putin, at the same time, wanted to do some "re-unification" of some previous Soviet territories including Crimea.
I'm also not sure why you're citing Belarus here. It was split off from the Soviet union in 1990 and governed itself the whole time despite being essentially a vassal state of Russia. Belarus has not ceded any land to Russia, either.
Edit - I see what you're saying about control or territory. If you want control, directly controlling the territory is better than having a puppet government. While Russia would have accepted a puppet government, as they have in Belarus (since there has been no good opportunity to go to war with Belarus to take it over), they had the opportunity to go to war for direct control and the West made it clear that Ukraine as a vassal state was not an option (see the 2014 revolution). If you think someone wants control, why do you think that they see $0 of extra value in directly owning the territory?
> It was split off from the Soviet union in 1990 and governed itself the whole time despite being essentially a vassal state of Russia.
It's obvious that Russia wants ukraine as a vassal as well. I would note that the invasion of Ukraine was launched _via_ Belarus, despite the fact that Russia does not formally control that territory. So again I ask, if Russia can get what it wants (which is _control_, not territory) without going to war, why would it do so?
Let's be plain - we are ultimately dancing around an empirical question, whether Trump will be hawkish or dovish towards Russia. Ultimately I think he's too chaotic for past behavior to be a good guide. So let's see what happens! I for one hope that you are right, but I think I have plenty of reasons to be cautious.