I don't think it means the game is fundamentally broken, by any means. I'm pointing out that the meta, which should only affect the top end groups, absolutely trickles down to groups where it is meaningless. In those two seasons, iw asn't running anything above the max key that gave gear. There is/was no comp that couldn't complete the keys at that level with a group of decent players. Outside of some very specific affixes, there was no combination of 1 tank, 1 healer, 3 dps that couldn't run a 10 (or the season's equivalent).
But the meta still trickles down.
People are not rational. They don't look at things like the top performing specs and see that they're only top performing because the group they're performing in is also doing XYZ. People just see that it is the top performing spec and want to bring it. Unholy DKs are/were top dps in Mythic raid groups (I havent checked in a couple of weeks) but only in the top 25% in Heroic groups. That tells me pretty clearly that either the top Unholy DKs are just much, much better than those in easier difficulties OR that those groups are doing something pretty significantly different with their Unholy DKs. My guess is those DKs are receiving PIs and Aug buffs very consistently and the ones in less progressed groups are, more or less, relying on themselves. Unholy is also more difficult than frost (or so I hear), so you'd expect to see lower skilled players performing better at Frost than at Unholy, so that's another factor. In reality, it's probably a bit of all three of these factors.
All that to say, Unholy might be the top DPS spec in theory, but there's a huge drop off in performance between Unholy DKs in the top groups and Unholy DKs in less progressed groups. That's important context, if you're trying to build the ideal group.
But people don't understand/internalize all that context, so they still fish for the meta spec, even when it's not really going to improve their odds of completing content. When there is imbalance at the high ends, it SHOULDN'T affect those who play at more reasonable difficulties. But it does, because we're human.
It becomes a lot worse when people perceive their own class as being garbage and start to reroll. This starts a cycle of "Class perceived as bad, decent players on that class reroll, remaining players are mostly bad, class is perceived as even worse because average performance has actually declined, because decent players swapped. Now the slightly less than decent players start to reroll..." and so forth.