So I've been reading through all of this and have a few opinions to throw in.
First (and most on topic), I think that it would be nice if there was some sort of soft indicator that would tell you as a DPS player if you were dealing enough damage to not be holding your team members back in whatever content you are playing. A simple indicator that only you can see somewhere (maybe a dot beside your party panel or whatnot). It would change colour as the encounter progresses to keep you informed.

Green: You are doing well. If you keep doing what you are doing, you will be able to make up for personal mistakes or even mistakes of others.
Yellow: You are still pulling your weight but you are making enough mistakes that one or two more big ones or if you start being even messier on your rotation you could start causing the group trouble.
Red: You are currently performing at a level where someone else is needing to perform higher than normal in order for the encounter to be beaten.

If everyone has a yellow dot, the encounter will still be completed. In fights without an enrage mechanic, the dot will never turn red. The reason I would like this is because if you are playing as a tank or healer... you know if you are playing well enough to beat an encounter. The party either dies or it doesn't. But when I play DPS I find it hard to tell sometimes if I am being a productive team member.

As for the other things discussed in this topic about elitism and gatekeeping and such... Here are my two cents.
Different game modes are targeted at different people. If you want to play the hard content, the content specifically made for people that enjoy challenge at a level where you need to optimize your play in order to be successful, then you need to make sure that you are playing at the same level as those you are playing with. The fun from THAT content comes from overcoming a difficult task with expert execution and tactics. You unfairly ruin other players' fun by not improving and playing at their expected level of competence.

HOWEVER that is not the only content in the game. A player should not be locked out of experiencing a piece of content entirely because of their ability to perform at the highest levels. They should still be able to play every raid/dungeon/whatever at a lower difficulty which requires only basic competency to complete, and all statistical and cosmetic item rewards should be the same across all difficulties. Drop rates can vary (reasonably, like 2-3x higher not 20-30x) but the availability of the rewards should not. Exclusive rewards for higher difficulties should be things like titles. An exclusive reward is important for a feeling of satisfaction, but the reward does not need to be something that excludes another player from advancement. Someone playing this content should be able to do the content without feeling anxious or feeling pressure from other players.