Fair enough. But you can look at every competitive game, either pvp or in this case, pve, players always want balance, and it still rarely happens. If you want to play one class and its competitive in support, aoe and single target and no other class thats fine but I think the evidence shows that more players are not only willing to switch classes when they rise to the top (which will always happen) but also that this model keeps more players interested in playing long term.
one thing players tend to forget is that blizzard makes all classes different is so players see clear differences between classes (so the game doesnt get stale) and more importantly, this allows us to develop synergies between classes to enable us to clear content better/faster. Aka meta
power creep is an inevitability. It allows them to focus on content, bumping difficulty and let the players decide how to use buffs/class synergies to best defeat that content. Its a win win.
i had a conversation the other day with a clanmate and he wanted to try his zdps support necro (perm land if the dead) with my LoN monk that is aoe oriented with cyclone strike. We were working it out and decided against it. Thats fun for me.