Vladivostok, Murmansk, Baltiysk and Novorossiysk all provide that, so that's not what makes Sevastopol so strategic. More to do with offshore natural gas discoveries being a threat to Gazprom and making sure NATO is never based there. Crimea being Ukrainian thanks to a drunken whim (legend has it anyway) of Khruschev wasn't a problem when Ukraine was still voting in relatively pro-Russian presidents and the east/south of the country dominated in the rada politically.