".make the Extra class test the only test."
If that were the case, in 1963, with my love of "tinkering", I probably wouldn't be a ham today. Most of us, (I'm assuming), would have gotten involved in another hobby.
There are so many different phases of ham radio that have very little to do with needing that kind of knowledge that I seriously doubt ham radio would survive. (I could be wrong) So many of us "normal IQ'ers" wouldn't pass such a roadblock, I'm afraid. (And, again, I could be wrong.)
I have no desire to experiment with UHF, bouncing signals off the moon, etc and consequently, I'd surely flunk such an exam. My love is just tinkering and not getting into lofty equations, etc.
(Besides, at age 88, where would I get the gray cells to even take such a test? LOL)