Between two Crimean wars and the Nazi invasion, Russia has about 1 million soldiers buried in Ukraine.
And there are people dumb enough to believe they are going to walk away from it without a real fight.
I don’t think Ukraine as a ‘buffer’ means much anymore in the age of thermonuclear warfare. But Crimea has somehow sacred sense, sure. A sort of Russian Alamo. Also Turkish slavers’ raids finished after capture of Crimea. Taking it was a sort of cementing the ‘Southern frontier’ meaning Russia only needs to focus on Western flank to feel safe.