I'm not worried about who benefits though, I'm concerned with who gets excluded. How many players lose access to this game because a few players on the higher end can't stand an occasional "slow" DF or have to kick someone?
You're just throwing numbers around to make your hypothetical problem sound worse now. You quoted 5% to show that if 5% of 22 million players is a lot of people. Yes that is. But your original idea of under 5% of people being so bad they're actually a detriment to the experience still just means 1 out of 20 instance runs which I'd say is well within what I experience (as someone who constantly caps their tomes and weekly lockouts). Are you that sure that more than 5% of the ACTIVE playerbase in NA is so bad they just cruise on auto-attack in dungeons and are so noticeably detrimental that they slow down an entire raid for faceroll content like crystal tower, OR are causing so much issues by being the one or two dead people in an actual attention requiring raid that they're wiping attempts?
Or do you just see bad (as in technically, not morally) people dying and wish that didn't happen ever because it's slightly annoying?
As a fellow anti-social recluse (lol Quarantine? you mean just the same ol routine?) I can tell you that yes, I have had bad experiences and over the 10+ years I have played this game I've experienced every BM thing you can imagine. People with horrible gear or cheesing item levels just to be in stuff, people who don't know mechanics and "share" the tankbuster or spread markers in endgame, people who throw tantrums at the slightest IDEA you suggest a change in their routine, the list goes on and on. But that's just people and NOT the usual experience for this game. I continue to experience a lions share of people getting encounters done in a reasonable amount of time and really.. not saying anything as long as I hold up my part. I tank, I heal (ok kinda badly), and I DPS and the stuff gets done and in 20 minutes I'm on the next thing..
My point being that yes there's crappy experiences to be had but they aren't nearly enough to straight up gate people out of experiencing this game.
Wonderful. I'm happy your friend overcame his disability and was able to meet your standards for gameplay (and my condolences for losing someone you cared about even a long time ago) . Some people can't. Some people LITERALLY CANNOT. They physically or mentally cannot do better than pushing a couple buttons every few seconds and some cannot remember or process the complexity of even basic endgame rotations. These people would be excluded with "hard walls" and are likely what "easy" modes are for just like added accessibility for deaf or colorblind people. I don't think "easy" modes are just to let leeches into peoples precious endgame optimal runs.
Being bad at the game or not being able to mechanically keep up is one thing , being outright rude is another. I don't abide being vile in any way and it has no place in this game. Don't expect people who feel you're trying to kick them out of the entire game to take it well though and I'm pretty sure you can kick people without telling them they suck first.