One of the major reasons why Russia is stated to be so much in the need of controlling the Crimean peninsula, is that their economy is restricted due to having too few options for sea trade, having very limited access to the sea. The north is frozen for a big part of the year, and Vladivostok is too remote, therefore they need another sea trading option.

However, even if Crimea stayed in Ukrainian hands and they blockaded the Kerch Strait and cut off Mariupol and Rostov, Russia still has over 400 km of coastline on the NE part of the Black Sea. Is that coastline so unsuited for building a sea port (or expanding existing ones, like Sochi)? Why is control over Sevastopol (in terms of sea trade) so important for the Russians, so they risk a potentially very costly war over it?