I am absolutely positive that they only gave him unique questline because of the fan outrage, they were genuingly surprised that so many people gave a damn and were constantly asking about him.
They were underestimating his popularity and overestimating Sylvanas, that everyone will be happy to have much more famous character taking his spot, to the point that even Pre-Legion event Sylvanas was already acting as if she was in charge.
I’m getting myself ready for next disappointment as the damage was already done, and his cameo likely will reach dead end.
He is just used as a plot device for that big death entity and nothing of value will likely come out of it.
As was already said above.
Too much was sacrificed to push this narrative, they had to destroy plenty of established lore about both horde and alliance to push this sylvanas “end game” and - while I can speak only for myself - I think that it was certainly NOT worth it.