So examples of how Blizzard makes it a toxic nightmare.
1. Keys cannot be reset. So if someone leaves then your key that might be a 10 which used to give the best reward for the week gets downgraded to a 9.
2. When the expansion first launched there was a bug that let you trash a Key for a instance you hated and reroll for a quicker run or better dungeon. Players loved this but Blizzard hated the idea that people do not love all of their dungeons so instead of taking it as a sign to redesign their instances to be more fun they blocked deleting keys.
3. To incentive people to actually do this content it is heavily reliant on gear and some tokens (atleast in the current expansion) as a way to get people to do it. Making it reliant on gear means that ilvl changes in an expansion means you literally need to farm the same item again. Right now if a 455 ilvl piece drops in Grand Cosmos and you are wearing 490 ilvl gear then you pass right?!? Well imagine and Ultimate Grand Cosmos where now that items has stats and slots that now makes it BIS. This problem is HEAVILY compounded in FF14 with a set gear vendor making this item or multiple items your new BIS over raid tier items. Now have fun refarming those same items when the ilvl goes up.
4. THE METAAAAAA. In WoW depending on the patch there is a meta group comp that automatically owns in Mythic plus. If you are not in the meta then good luck getting into a group. Sure some hardcore raiders will say it does not matter but these guys will also farm 16 hours a day to get world first on an Ultimate. So problems that are obvious in raids get compounded far more heavily in Mythic plus effectively killing classes.
5. In WoW it has led to guilds not seeing much point to any Normal or Heroic content and then not having the skill/people to do Mythic so they have no real point in logging on. So imagine you have a static that is working on a Savage but now you have Grand Cosmos savage that rewards the same or higher gear in an ultimate that can be run in a minimal amount of time. What point does the Savage raid serve anymore especially if you know if you are not going to do Ultimate.
6. Making gear the incentive to run on the hamster wheel the RNG has been turned up to 100 or else you would farm it quick and have no reason to do it again. Another cheese exploit around the system has been to bring all people in a class who wear leather so you get more leather drops.
Mythic has multiple knock on effects once you add gear rewards into the equation. Only thing that makes sense is achievements, currency, minions, mounts and glamour.