If Ukraine takes back Crimea they will have to destroy the fleet, which would be awesome but hard.

Why would they destroy the fleet?

Russia does have its own black sea coast and a naval base at Novorossiysk. It's not as big as Sevastopol and of course because of it's location on the far eastern shore of the black sea is less advantageous strategically, easier to exert control of something when you're closer to the middle of it instead of on one side essentially. In addition there is nothing stopping Russia from sailing those ships out of the Black Sea through the Bosporus strait and to other Russian ports. The treaty with Turkey prevents warships entering the Black Sea during a time of war that aren't usually stationed there, they would have no problem letting Russian warships and subs leave.

When Ukraine hopefully retakes Crimea, either through force or in negations, Russia is not going to destroy their Black Sea fleet. They have other ports on the Black sea and could always take their ships to their other major naval bases in the Arctic and Pacific regions.