Russia may only really want the more ethnic Russian areas, the rest of the Ukraine would not welcome him and would be a financial strain, he may see no benefit to staying there. I would not be surprised if his main goals were always to stomp out a bunch of the Ukrainian militia that have been bombing Donbas for 8 years, which required going past the Donbas border to get at them. I think also he wanted to restore water flow to Crimea, which has now been done. Long term, it think he will withdraw out of any territories that are unfriendly to Russia and try to keep all the ones that are like Donbas and Crimea and possibly a few other mostly ethnic Russian areas on the east. It's likely now that Ukraine will be even more antirussian than ever so those areas would have a harder time of it if they stayed with Ukraine too. I don't think Eastern Ukraine and Western has any real hope of getting along with each other at this point. I don't see any other way than for Ukraine to let some eastern areas leave.