Challenge = fun is the core concept of games. Developer gives you problems and limited tools to solve them.
Take for example a simple problem of going from point A to point B:
1) You can run on foot and fight through various mobs you meet (risky, engaging)
2) You can run on foot on the road and meet no one (safe, boring)
3) You can run on mount through various mobs and occasionally get knocked down and forced to fight (risky, engaging)
4) You can run on mount on the road and meet no one (safe, boring)
5) You can fly on mount and meet no one (safe, boring)
See a pattern here? (not trying to turn this thread into flying debate, but it's the most obvious example).
When you have a tool that allows you to skip problems that developer put in front of you - it becomes less fun, especially when it takes less time to ignore problems, than to actually deal with them (in case of doing stuff on foot you have a choice of going on the road (long path) or going straight (short path with occasional fights)) .
Then you have to account boredom factor - when you overgear content (or do it for 100th time) you simply don't feel engaged by the same old problem developer put in front of you - you solved it numerous times and it doesn't pose any interest to you, off course in this case the "short and easy" path is going to be more favorable for player, because they can skip boring problems and go to fun ones (or, which is often the case - skip straight to reward).
Blizzard should (and they do) cater to all their community, but they can't please everyone, they have to find common ground, and more often than not - they do find it. With literally any change there will be people who are angry with it (regardless of how irrelevant it is), remember outrage over selfie camera patch? It has literally zero impact on the game, yet people were raging over it.
Blizzard also have to keep clear borders to how hardcore hardcore players can go, there is obvious difference between a player who plays 6 hours a week a player who plays 60 hours a week, and blizzard have to make sure, that they don't get too far ahead, splitting community too much, stop this unreasonable "3k GS sart +0" meme raids and stop hardcore requirements from slipping into "casual" community in general ("benched for wrong legos", "benched for playing survival" vs "benched for shitty performance in raid")
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You can put LFR and powerful rewards in the same sentence since the throne of thunder.