Exactly then it was possible since
such system let hybrid classes exist, and what became of them now? What become with all various
characteristics that not so much reinforced "absolute" indicators of characters strength, but more players' "
role"
choice, even if taking into account "holy trinity" (also, this isn't ultimate truth because I repeat - don't forget support, which is 4th)? That is, any class was able to at least partially
fulfill not only role assigned to it by devs, and therefore even "holy trinity" based encounters design didn't severely limit group as a whole (unbalanced from
current point of view) abilities.
Even DK, which appeared in WotLK (MoP monks weren't already), still confessed this "religion", because tanking/dd role completely depended on character RPG customization... but now...
everything depends on chosen "spec" and no one care what player thinks about it (the whole "
role" RPG part is
integrated inside it and you can't change anything), we have only one opportunity to play -
the way it was decided by Blizzard and nothing else (by the way, "gladiators" part was the only bright ray in dark realm of
specs after "design cataclysm" that destroyed everything, but it was just an exception, and what became of them only proves my point).