If Ukraine is ever successful with a ‘push to the sea’ anywhere near/far around Mariupol, cutting off Crimea other than the Kerch bridge, then I’d say even then they’d leave it standing.So they don't quite have the capability or capacity then.
How many troops does Russia have in Crimea, with Kerch down, they are trapped - which might sound good, but it might go a few ways:
1. They become determined and take it out on the remaining Ukrainian Crimeans.
2. They surrender en masse causing a large logistical headache for a stretched, albeit victorious, ukrainian army
Leaving the bridge there, means the Russians can ‘escape’ but in complete disarray, abandoning their heavy equipment, artillery etc, just fleeing with what they need to survive, intermingled with a Russian civilian population also fleeing.
I don’t believe Ukraine doesn’t now have the capability to knock it out for a longterm period or destroy it, they are just choosing not too for whatever reason which is outside any of us’s knowledge.