As long as Severodonetsk is not encircled, Ukraine will still be able to hold onto, resupply, and fight back there. It won't be the same as Mariupol (as long as Ukraine holds the Donetsk River) even if the Russians flatten it. Which I think they've already basically done.
The Russians can't seem to get across any contested river without Ukraine completely demolishing their multiple attempt(s), so, that may be as far as Russia can go in the Donbas... halfway into Severodonetsk.