In my opinion, they should work together and come up with something.
Maybe if Blizz takes over the servers, add them to their existing servers and just ask for example €20 to “unlock” them (Won’t change the monthly costs)
In that way, people that quit WoW because they ‘miss the feeling of vanilla’ can play on official…

They actually don’t profit from other people’s work imo
They just created a way for people to feel the experience of vanilla again, because Blizz couldnt do this.
In my opinion they should work together and maybe make it pay-to-play if they want, so people who quit can come back and play their favourite expansion