Comment by int_19h

Ukraine has a severe manpower problem, while Russia hasn't even implemented full mobilization yet. They can keep grinding down the existing defenses until there are simply not enough Ukrainians in uniform left to hold the tide, and then things would break down pretty quickly in the absence of external support.

They would still have to contend with an insurgency on occupied territory, but that is something Russia has considerable experience with, including Ukraine in the past (mopping up the remaining nationalist resistance after WW2).