They really don't want to be seen as an aggressor though, even though it may be justified as some sort of possible payback. Tactically it would be a mistake, and ultimately they would never be able to hold it, as it's a lot harder to attack than to defend. NATO will have some sort of contract saying the weapons given can only be used as defence, and within the Ukraine border.
DMZ's or contested zones rarely work, and just drag things out, it would create a permanent enemy, where as a "normal" Russian loss/ retreat may be enough to get Russia moving in the right direction again.
Ukraine going in on Russia would be suicide, but would also make loads of Russians think "maybe what Putin was saying wasn't all b***ks", don't give them that bone to chew on.
Zelensky's not daft though, and they've got nearly everything right tactically, so they won't make this mistake, it would be a massive one.
Get Russians back in Russia and they could tear themselves apart/ split up into different areas, or overthrow Putin and move a lot closer to the West, which would be good for them, and us.
I think some of the post is getting ahead of itself, but looks to be going in that direction.