It's not that people don't think really bad players exist. It's just we don't think these running at 5% people are as prevalent as those arguing for "standards" would have you think.
People get on the internet to complain. With a population of millions it'd be surprising if people didn't have stories about meeting a bad player every once in a while. I'm not sure if you've actually run through that thread, but it's people complaining about all kinds of problematic things. I've run with people who barely make Ilvl in 30-40ish dungeons, people who refuse or don't know how to AOE, and people who outright will eat every single mechanic. This is still not people running at 5%. The dungeon or instance gets done a little slower but it's at most a 20-30 minute thing. It's not great but it's also not the brain destroying ordeal others make it out to be.