I'll disagree so, so very much with this and I am certain this comes from someone who hasn't played the game for a while or played very casually. Yes, doing quests and just roaming around has became way easier than before, however the game has tons of room for anyone who actually wants a challenge. I don't want to go super deep, but high end raiding is still alive and finally started getting monetized/sponsored plus you can always push mythic+ dungeons. It has to be noted, that all of this is group content and what WoW lacks right now is difficult solo content and I'd give you that.
Basicly right now the retail WoW can be played as time-intensively and on whatever difficulty you want. There is always something to do, however you hit diminishing returns quite soon. There is tons of hard, engaging content to be done with friends.
I think Classic is kinda fad, something a lot of people think they wanted, but after some time they will realize they did not. Of course, I might be wrong in which case I am happy they got a completely new game to play and hope they enjoy it as much as I enjoyed the two years of WoW Legion(had to stop, because I did not have time to play at the level I wanted).
On the other hand, I can't wait for Warcraft 3 Remastered, that will work by default with most of old custom maps. I am taking a week off for that. :)