Ok, but how do ppl improve then?
Suppose someone has an issue with +10, but finds +9 easy.
This is very likely, because of the affix at +10 which makes both trash and bosses deadly.
It would be more productive to let ppl try keys which they find challenging, instead of keys they find easy.
Btw in LFG, ppl should be required to explicitly set their goal (Beat timer, completion, learning), so ppl will queue with similar mindset (nobody is surprised if someone leaves a beat timer key if the timer looks bad after a wipe).