The word average (skilled) pilot should be avoided for the purpose of this discussion. If we are true to the mathematical meaning, adding even a small number of extremely well proficient people to a group, the previously "average" pilots of that group become less-than-average.
Less than average skilled sounds like "not competent enough to fly an aircraft designed for an average pilot". Which itself is an element of the debate, thus the word "average" makes a big mess of everything here.
My suggestion is that "the average pilot" in the certification realm means "any pilot who passed the licence proficiency criteria with at least bit of a margin, is able to achieve that or better performance continuously (as his experience builds but memory fades), and is not having a bad day"