Tools they could add besides DPS meters... (For endgame)

[Sealish]

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I think that the point you are trying to make has merit in some circumstances, but not in all. The level of performance that someone is expected to play at is entirely determined by the content they are doing and who they are doing it with.

Duty finder? As long as you are actually playing and not just AFKing you're good. The absolute bare minimum that everyone should be able to expect from everyone is that they at least perform actions with the goal of clearing the content. The bar and expectations for this is very low.
Party Finder that lists content but no other expectation? You need to be able to perform at what you above lay out as the minimum determined by the Devs. Mechanic skips are not required, but being able to play well enough to clear the content is. This is not a learning party but a clearing party and by joining it you are saying that you know that you can clear it.
Learning Party? No solid expectation other than that you are actually trying to improve and are open to constructive criticism. You need to WANT to get better and actively try to.
Party with expectations listed? You need to be able to meet those expectations. The person/people who made the party are looking for a very specific playstyle that they find fun. If you are joining the group and cannot play in that way, you are in the wrong.

Just like sports, videogames are games... games with differing levels of play for different types of people. All players are expected to play at a level determined by a combination of the content level they are playin in AND the stated goal of the group of people they are playing with. Trying to enforce excessive expectations on someone when the expectations were not laid out at the beginning is wrong, and so is joining a group with listed expectations when you know that you cannot meet them.