in a few expansions, there will be no need to do new expansions. interested players will play anyway, whether or not there is new expansions. the economical return to produce a big expansion won't be worth it compared to the added engagement/new players. they might release a wow 2 eventually to appeal to the bulk of the new generation, because those are not really playing wow as much as the previous generations did.
they will keep the game running forever, add multiple classic expansions for maximal player interest, and find ways for it to cost as little as possible for them, potentially making the players pay themselves or share their computer resources. wow will likely be forever profitable, there will always be some people willing to play, but not as much as when it came out first. new laws in a few decades might make it illegal for blizzard to operate the way they do (personal data and virtual ownership will be a much bigger deal than now).
therefore the level cap won't be an issue, it could stop at 100 or something.