Russians will move their border as far as you allow them to. I know their mentality, for I am one, and I grew up in a town full of army and fleet people.
I saw many types like that, before I got out of the country. Those will keep bully you more and more to see where you set the line, and punch them back.
In their calculus, being punched back is not bad at all, if before that they were able to punch you "for free"
Russia's Crimean desire is Russia's desire for land and spoils of war. Nothing more nothing less.
Crimea just happened to be there with some historical justification with which they came months into the conflict. Russia was plainly unable to advance any much further beyond the line where Ukrainians were scared enough to fight them seriously.
Russophone sentiment? No, nonsense, otherwise they wouldn't be occupying Ukrainian majority regions, or allow ethnic Russians to be made hate crime victims in their own backyard (Armenia, Kazakhstan to some extend, and ethnic autonomies in Russia itself)
Their desire for Crimea is not much different from their desire from getting a cut of land on the Rheine, or french Riviera, or even your capitol hill if you allow them to.