It only makes sense that the Ukes blew it up. One reason Russia started this war in the first place is to secure a land-based link to Crimea (the Mariupol line) and they’re going to end up with it. Ukraine is the only player with a motive to prevent it from happening.
Your scenario doesn’t connect. How does this help Ukraine take Mariupol?
The place to cut off the Russian held Zaporozhia/Kherson territories is Melitopol, IMHO. That’s the chokepoint.
It does help Russia hold these areas however, by impeding a crossing of the Dneiper. The docks, jetties, and connecting roads on the Russian side are being washed away, complicating the use of ferries. The East bank will be an extensive muddy mess. And the now-irregular flow of the Dneiper will impede the establishment of pontoon bridges, even alongside the Antonovsky bridge.