WoW expansions sell more than 3 million in the first day alone. You are delusional if you think they will stop anytime soon. Even if active players were to drop, just the "box" sales alone garantee profitability.
Dropping below acceptable numbers would be a drop in sales in the 80 to 90% order. We are not even close and yes, the sub numbers will be include cause classic is just another feature of WoW.
This is exactly why classic is part of the live sub. Theres only one WoW. classic is just a bonus mode being offered, just like pet battles, for example. Classic will not see the content support live has. It will come to a halt eventually and people will lose interest. Just a few purists will stay behind. But, having the live game in that sub too, means those bored will have somewhee to go to for new content without losing their nostalgia game. They don't pay more or less for it.