Time was the Russians would hold a position at any cost -Port Arthur '05; Brest Litovsk '41, Sevastopol '42, and Stalingrad '42-3, to delay the other lot, but this pull back makes sense. I doubt the Ukrainians can operate at the scale necessary to cross the Dnieper and take the approaches to Crimea. I suspect Russian strategy has been reduced to holding on longer than Ukraine and its allies can keep going. They are masters of the frozen dispute. It is a pathetic return on their original plan as it leaves them broken, weakened and shamed, but there is sense to a strategy based on growing Western fatigue.
Sooner or later, serious negotiations will begin, but that's like saying sooner or later the rain will stop, when the important question is when. And sooner or later, the salient solution will emerge: Crimea to Moscow; Donbas to Kiev, and the West rebuilds Ukraine. But no one's near accepting that yet.