It may be 'just' Mariupol, but I'm suggesting that the logistics of continuing that attack is straining Russia's capability. Possibly a combination of supply problems and things like having any troops left who aren't becoming too aware/bothered that they're bombarding civilians rather than whatever mythical Nest Of Nazis thing they're dressing it all up the culural, civic, educational and health buildings up as.
And if they can't 'legitimately' win Mariupol (ideally with handy (imported) crowds cheering the 'liberators' from Mother Russia) then they run short of the kind of spin they need to feed the state-media to keep the masses on their side (or cow the other masses that aren'tbon their side, all those people losing johs/gigs/liberty for voicing contrary opinions).
I would not wish my worst enemy to be in Mariupol right this moment, but I doubt the situation gets better if 'signed over' to Russia, and everybody (on both sides) involved in any such talks will know it. It'll be bad if they don't surrender (increased bombardment from a distance, with whatever resources can do that without starting to see the whites of the residents' eyes) but that loses them the 'fair' victory and they need to explain more about 'suicidal buildings owing themselves up', en mass.
(PPE: Yeah, Eric the ninja has some of it. And, like I said, I'm extrapolating from my armchair here. The true history of the future may surprise me, it's hardly like I've got an accurate track-record here.)