Russia has warm ports on the Black sea such as  Novorossiysk. Black sea fleet is not really important to Russia or Europe for that matter. Black sea cut be closed in Bosphor straight near Instanbul. Russia has warm ports in Syria, Kaliningrad and many other places. Baltic fleet is also not most powerful fleet of Russia. Baltic sea is lake that can be close somewhere between Germany and Denmark. The powerhouse is Russian North fleet where most powerful ships and submarines carrying nuclear weapons are located.  But you are right, Russia could not except that Crimea will remain in Ukraine – possibly in NATO in future.

The entire region from Baltic Sea (Estonia, Latvia, Lithuania) to Black sea (Belarus, Ukraine, Georgia) is a buffer zone between Russia and Europe. Hence, there is a lot of geopolitical interest between West and Russia in this region.