What's an MMO really? It's not a trivial question.
Technically MMO means an online game that can be played by a very large number of people simultaneously.
Fortnite is an MMO by that definition. So no, WoW doesn't need to die for another type of design to come along and be successful.
If by MMO you mean something like WoW? No. As others have said MMO's have moved on to different designs and types. WoW has its niche, a very large one. It was a freakish success from the start and no one should expect any other MMO like WoW to be launched that charges $120-$180 dollars a year to play it and be that successful.
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Yes it is. A lot of people will play both in differing amounts according to their preferences. And why shouldn't they? Your idea that Classic is going to be a WoW killer is just as silly as all the other MMO's that were going to come along and be WoW killers. I don't understand why you or anyone else think it's a fucking death match competition in any case. There's plenty of room for both as they are under the same subscription plan.