1)
Classic servers will have higher pop than live servers because of all the returning non currently playing players and those that will go from live to classic.
2)
Blizzard seeing that, that they are making more money from classic than live, will start to slowly shifting their team and resources into classic.
3)
Main game having lower and lower players is slowly dying out
4)
Blizzard going full production on classic, like in their announcement trailer all the expansions are 'rewinded' and do not exist, basicaly classic becomes an 'alternate timeline' and now they are starting to make new content for it basing on vanilla mechanics.
5)
Classic becomes the 'new' main wow with new expansions and QoL from over the years but with vanilla mechanics in-tact.