I guarantee that even if WoW got back upto its peak quality, with true fun and engaging content, it would still not be able to sustain more than 8M subs for longer than a month or two. It would shortly fall back down to sub-5M. It's because its a 15 year old game.
How many players will Fortnite have in say, 15 years?
Whereas if WoW 2 was made, all brand new, it would probably sustain so much hype (which is the #1 factor for any games growth nowadays, with social media publicity and any publicity in general) to gain upto 15M subs within first year.