In terms of the NP training model being unproven/untested, I feel like you're implying that our current training model was proven through some sort of carefully considered double blind test. It wasn't. Premedicine was created when a bunch of women wanted admissions to Hopkins and medical school suddenly morphed from a undergraduate program into a graduate program. Post graduate training has slowly expanded from a single Intern year into 3-12 year nightmare of residencies and fellowships we now endure based entirely on the opinions of program directors who, BTW, get to use residents for free labor for the duration of their residencies. Considering how unscientific the development of medical training has been, I feel like it's a little unfair to turn around and ask NPs for double blind trials before they can practice.
As was already said, the US medical training system has been proven over time to produce superior physicians. Burden of proof is on midlevels if they think they are in some way equal to the level of aptitude of physicians.
Prior to the formalization of GME, if you wanted to be a surgeon, you became an apprentice of a known surgeon. Graduate training still existed, just in a different form. And of course, now we can do things we didn't even dream of in the 1960's like operating on fetal hearts in utero. It stands to reason that super-specializations like these would take many years of toil to hone.