How do you know they're performing at 15-20 compared to the entirety of the player base?
If you're making that assumption off what gets uploaded to FFLogs, you're not getting a true sample of performance from the player base in general let alone a complete report of all player performance. All you're seeing is what someone bothered to upload. Considering the logs are mainly relevant to the high end raiding community, the majority of the community aren't going to be recorded there because they're not doing high end content or they're doing it casually in pugs where there's unlikely to be anyone recording. Those 15-20 players might actually be 60-70 players if that data was getting collected from everyone.
I get that the thread was intended to address what some players perceive as a problem. Go back to my original response on the first page of the thread:
You also don't know much about people if you think everyone is able to understand what they're supposed to do just from reading tooltips. Not everyone has critical thinking skills and yet the game is here for anyone to enjoy regardless of what their skill level is. There's a reason so many players ask for guides - they need someone else to figure things out and explain it in language they understand.
There are a lot of players that won't even ask for a guide. Why? Because becoming a better player isn't necessary for them to enjoy the game. Their enjoyment is coming from sources other than performance.
The game is here for everyone to enjoy regardless of their skill level. SE places no burden on anyone to "git gud". They've even gone as far as adding the easy and very easy modes to solo duties so no one will be blocked from MSQ progress. If you're running into someone in EX/Savage/Ultimate who can't cut it, by all means remove them from the group. Maybe if they get removed often enough, they finally have the motivation to become a better player.
But it's not something that can be forced on players.