And the legitimate government can operate in exile, ready to return if it becomes viable. Inusrgents fighting on their behalf could coordinate with them. I'm very out of my depth but it seems like this must be a common scenario, particularly when the displaced government is legitimate.

Certainly, but that is fairly pointless if the existing government still controls 90% of the country.

I think the psychological effect of losing Kiev would be much lower if they planned on that from the start too, but I guess that bird has flown.