I just came back after a break (guild fell apart at end of NH) and I have to reroll for my new guild since they have 5 of the class I was.
The legendary grind is soul crushing and I couldn't imagine any returning players willingly doing this just to get into raiding right now. Previous expansions was a lot better feeling, you could at least know in 8 weeks you'd have the ring/cloak with minimal grinding, and you would "definitely" have it by then. This new system you have to just grind against an endless wall with no idea when it'll end.
If this guild told me I needed better legendaries than the two garbage ones I got after 3 weeks of hardcore grinding, I would've just quit again.
I think that's a good real example of where all the returning players went.