I'm not entirely convinced that they're dying, we've just put the days where one single MMO overwhelmingly dominated the market behind us. FFXIV, WoW, GW2 and ESO are all doing fairly well for themselves and receive ongoing content updates and they're far from the only ones.
There's a few MMO's set to launch at some point in the future and eventually one of them will capture the lightning in the bottle effect and do something that brings genuine innovation to the table.