A very good point, there's a real practical problem in place. I actually would argue that the fate of an area should largely be decided by the people who live there. But like I said, I don't believe Ukraine's governing elites have any interest in letting the public decide this.
A hypothetical, just to probe Russia's position on this:
Russia is offered this deal. The war ends. The occupied territories are handed to a neutral international administration, Saarland-style. After some time, the people there vote fairly on whether to join Russia or Ukraine, and the territory goes to whichever side wins.
Do you think they would accept?
(I realize a lot might depends on the details: one vote for the whole area or separate votes per oblast or raion, and whether pre-war residents get a vote or only those living there now. But I'm deliberately leaving that open)