I don't think you understand what i mean.
Lets say, e.g. if wow had 4 million active subs, after classic was released, 1 million of those subs went to mainly play Classic, plus 1 million return players etc. After TBC, wrath, cata, mop, there will be, let's say, 2 million from the remain 3 million retail players go to those different expansions, because just as you said, people prefer differently things.
This will result retail have much less active players. The more expansions there are, the more divided is the player base, and the less active players are in each of them.
Ofc, in theory, it won't change the total sub of wow, but it can possibly results in each version expansion only has few hundred K active players, unless you think those subs can play both old expansion and retail actively.
If retail has fewer and fewer active players, it will has less and less activities, more and more dead servers as a result. And if retail only counts for minority player base and sub income, there is less motivation for blizzard to make it better.