Sounds like the goal is for Putin to take Kyiv and replace the government with installed Moscow puppets.
That also would result in a widespread and ongoing insurgency to fight, so yeah, they'd probably need to maintain a presence.
I'm no expert, but I think the plan is that he takes Kyiv so quickly and decisively that the West stands down rather than to try to roll that back. Which they probably would, as nobody really wants to go to war.
Also, I think there will be a lot of hesitation by the West to very visibly back Ukraine with arms and supplies, without knowing if the Ukrainian defense is just going to roll over dead. That would be a triple value for Putin taking Kyiv, as he could declare proxy victory over the U.S. and Europe.
And the US has spent untold treasure and lives trying to prop up people who don't really want to fight on their own behalf.
That's what makes this so compelling to me...the longer and harder Ukraine stands, the much more likely the West is to really back them up. If the fall of Kyiv is inevitable, nobody in the West is going to want to risk too much over it. But if Ukraine holds long enough, the calculus might change if the West thinks that with enough support, the Russian invasion could be broken, either directly or from internal Russian pressure.