Are they actually though? How many of those people keep up their subscriptions the entire time?.
I think it's pretty easy for them to check. If they go by how WoW figured it out it goes something like amount of people clearing the content vs the amount of people attempting the content vs the amount of people not bothering with the content when they could if they wanted to. You'd be silly to think they don't make their gaming decisions based around that ( with plenty of verbal/written player feedback as well of course). This is how it's been since day one, especially after having to recover from 1.0.
Of course, they never share concrete numbers with us, but they usually tell us about some of them after the fact (like how Titan HM completion rate vs attempted was in the single digits pre-nerf or something ridiculous).