I think I see your stance on this a bit more clearly. It's about accessibility. I've no counterpoint on that, nor would I want to make anyone feel like MMORPG's are a space for the elites only, the genre only improves with new people lest they stay the same.
My counterpoint is this though; much like most of the internet you're not going to encounter people who are going to like/care/want to put up with someone who is underperforming. If you're queuing up for Savage content you probably don't care if the White Mage is hungry, or if the Paladin's AC just kicked off, you care about if they are holding aggro and healing. It might be old fashioned, but you can't expect others to cater to your specific needs. We all have our circumstances and with all due respect, if someone is that worried or anxious about running dungeons then I shall borrow your solution, and say they should use DF to make such a group.
I agree with the sentiment that you can control 10% of what happens to you, and that 90% of all things is how you react to it, but that same sentiment needs to be put on the people who might lack in ability as you stated. I don't think it's a valid argument to state that because someone is playing a game, the time they spend in it is useless because it's not work and it's a choice. I think that their fun is derived from the time spent in game, doing what they enjoy and for some it might be running dungeons.
I sort of wonder if you'd say "just leave" to someone if they were being antagonized by 3 others for falling into the free-cure trap. If I am understanding, it sounds as though you support the idea that we must move as fast as our slowest, and while touching, it's not reasonable to ask that of strangers whose time is just as valuable as yours. If we're going to tell people to make "Optimal Parties" in DF, I think we should see "Casual Parties or Laid-Back Parties" in DF as well. I don't think it's any more reasonable to ask strangers to be patient with you than it is to ask for the healer to keep up.