Right. Reminds me of the old joke, "what do you call the person who graduated last in med school?" Doctor.

Here's the thing about that joke: you could tell it no matter what the standards of people at medical school, and no matter what the difficulty of passing the examinations.

If medical schools are good at accepting only people who are likely to make good doctors, and at failing students who turn out not to be good enough after all, I'm not all that bothered by the prospect of being treated by someone who just barely passed. (Of course that's a big "if".)

The same goes for the OP here. How do you define that ten-point scale? If, e.g., "6" means "doing a good job, but there are other better people around" then firing your sixes may be a lousy idea: perhaps all the better people are much more expensive and you get better value from a cheaper 6; perhaps all the better people are off working for other companies; perhaps in this particular job the difference between a 6 and a 10 doesn't really matter all that much, and the cost in recruitment effort and training will outweigh the benefits of firing your 6 and getting someone better; perhaps the person currently in the job is a 6 who's working his or her ass off to turn into a 7, then an 8, then a 9, then a 10.

... Or, of course, perhaps not. The original article does specify that it's talking about people who "cannot do the job". But what sort of scale are you using where 6 means "cannot do the job"?

I think there's something badly wrong with the idea that Only The Best Will Do. Because (1) for a lot of purposes, someone who's not The Best can still be plenty good enough, and (2) there aren't enough of The Best to fill all the jobs, and (3) if enough companies start firing everyone but The Best you're left with a large number of people starving because no one is willing to employ them to do the jobs they could do perfectly adequately if employers weren't all insisting on getting The Best. Oh, and (4) guess what, these companies generally aren't keen on paying every single employee a superstar salary.

There are jobs for which you absolutely need the very best people you can get. In, for instance, a new startup, that might be all the jobs you've got. In that case, by all means insist on hiring only the best people and firing anyone who turns out not to be. But in that case, you'd better also be offering rewards commensurate to your demands.

  ...you could tell [that joke] no matter what the standards 
  of people at medical school, and no matter what the   
  difficulty of passing the examinations

My Mum (dying of refractory metastatic triple-negative breast cancer in her lungs) is being cared for by an oncologist who's a six. It isn't pretty.

I'm sure it isn't (and, my condolences; that must be awful for you and even worse for her) but ... would you rather she didn't have an oncologist at all? (That's what happens if you just get rid of the sixes and change nothing else.) Or that she had a seven, who's working 80-hour weeks and working like a six on account of exhaustion? (That's what happens if you geet rid of the sixes and expect everyone else to pick up the slack.) Or that she had a seven but someone else with similar problems got her six instead? (Actually, I bet you would prefer that, and I don't blame you, but it wouldn't be any improvement for society as a whole.)

Having everyone be more competent would be a wonderful thing, but that isn't something you can bring about just by firing all but the best.

Oh, we're firing her, all right, and going with an oncologist who comes highly recommended. Some kind soul like you can tolerate her incompetence.

"But what sort of scale are you using where 6 means "cannot do the job"?"

Maybe 1-5 are people who are actively harming the company?