I agree. But they don't need to give up for a puppet government to be in place. They need to lose control of the military and communications infrastructure. If the government is far to the west or in exile, it's hard to see how they would maintain control.

And I also agree that a new government would need a substantial Russian military presence. It wasn't that long ago though that a substantial voting bloc supported pro-Russian parties. It's likely that Putin calculated that they'd only get problems with western Ukraine. That's likely been badly misjudged.