Its not a case of being BiS or bad.
The further to the edge you get, the more dps, utility, and comps matter, this is due to the ramping endless difficult the game offers.
If for example.
The dps variation between classes is 20k.
You have to accept in a endless ramping system eventually that additional 20k per player will be demanded by the game itself.
Weather u like it or not.
WoWs end game is a dps check.
M+ theres a mimimum group dps requirement to time a key.
In raids theres a maximum time before enrage.
With endless scaling difficulties this means the games built for content to become too difficult i.e the scaling will eventually demand the absolute meta.
And will eventually one shot you on a auto attack.
To ignore that is just ignorance im afraid. If the difficulty scales endlessly. The more dps every level requires. With that said its very logically forseeable.
Also. Ur ignoring difficulty.
Its easier to time a +20 key on a meta class then a subpar class, thats factual. So if ur simply not skilled enough for a +20 players can rely on meta classes for accessibility.
Your correct, any specc could time a m+20, and any specc could accomplish raids, what ur ignoring is the vast majority of thekr raids and groups are meta.
If u stacked a Vengence DH, 2 Frost DKs and a survival hunter with a holy paladin into a +26 key. The likelyhood is unless ur absolutely exceptional players. U dont have a chance to time such a key.