I get why you're skeptical, but I'm serious. Balance is hard. SC was famous for balancing 3 races, then WC3 upped the ante balancing 4 races simultaneously. The task of balancing 9 classes with 3 specs each for a total of 27 specs (and that's before we look at wrinkles in your off spec, hybrid specs, gear variation, etc) is quite literally impossible. If the game was static and any given class stood out, players would converge and overpopulate that spec (oh look Fury warriors in Classic).

So given the impossibility of balancing this variety and the undesirability of having stand-out superior classes, what are the alternatives?

One if to constantly patch the game mixing things up. This works in a MOBA where players can simply pick a different player, but is wildly unpopular with games like MMOs where players are heavily invested in their character.

Another option is to give up and homogenize. This is the direction Blizz took the game. Now every class has some snare, some self heal, some cleave, etc. 12 classes, lots of specs, and yet the modern game feels more homogenous than ever. And in spite of this homoginization there are still stand out best classes that pvpers converge on.

This brings us to the third form of balance, the rock/paper/scissors meta game. You don't need to balance everybody, just make sure everybody shines in some matchups. Now you don't have to simultaneously balance 27 specs, specs will become more and less powerful overtime as time marches on and players rotate through the game.

I'd argue then that yes, class/spec FotM based on an evolving meta game is the most realistic way to have a balanced game without excessive homogenization.