Yes, people will play the way it suits them and the non dedicated will fall off, of course.
But I fear that the dedicated ones, the ones ready to play countless hours and to do all that vanilla required you to do, will be too few.
Back in Vanilla, most people wanted to go far, to achieve success and dedicated themselves, just like people do with retail now. Since this will only be an optional game and since retail remain as the prolly more prestigious and more success driven game, this will make Classic server have less quality than vanilla.
I think getting World first in BFA will be more wanted than getting World first Naxx in Classic, and that's trouble, cause we won't see the same passion, dedication and drive in players towards classic as they had in vanilla.
TL
R; I want Classic to be seen as Vanilla and for the players to be as dedicated as they were in 2004, to give Classic more quality and to get a better vanilla-experience.