Exactly this. It's not like WoW has never been like this. Even in vanilla it was like: only tank is warr, every hybrid is a healer etc. It's not a new concept, yet people seem to forget this, even the big vanilla fanboys.
I attribute that to players being more 'casual' in the past, so they didn't care so much.
Nowadays the situation is pretty balanced. There are very strong specs based on current encounter+class design, but you are not forced to do it that way. Unlike you were forced to tank as a warrior only back then. The difference between classes are minimal relatively to what it had been in the past.
This should be a relatively easy concept to grasp for someone who plays class-based online games.