This is such an overstated issue. There's not really that many MMOs. It's hard to say, but I would estimate that there are ~600-1000 MMOs total. Many of them are quite simple (like Club Penguin, MapleStory, ...) or already have third party servers running (WoW, City Of Heroes, ...).
Also, your example doesn't really make sense. First, no one is arguing that companies should right now go revive all past MMOs. This is something that would affect future products.
Additionally, why are you making a sequel to a product that already failed?
Yeah I remember this argument from decades ago where people claimed that WoW required specialized hardware to run that would be far out of reach of community server operators. What a bunch of bs that turned out to be.
What do you do for, say, a failing MMO built on a proprietary game engine you're trying to make a sequel for?