Please see my response to the person you're replying to.
In my last reply, I’ve already listed multiple reasons why we don't advertise the precise criteria. Did you see that?
Elsewhere, in more purely scientific settings, I definitely have discussed the exact criteria and the evidence for them.
Furthermore, these are arguably proprietary business trade secrets, yet I've made them public in order to try to advance this area of science.
I think your comment ignores the plethora of evidence supporting donor-quality hypotheses. Much of it was presented in the OP, and covered the permanent extinction of our host-native microbiomes, along with the exponential rise in chronic disease.
Your suggestion seems to be to “try to find a plethora of plant and wildlife species in a forest that has been burned to the ground”. Whether you can piece it back together is unknown, but I don’t think that’s the best approach to take right now.
Also, one of the major problems is that most people are not even bothering to look for high quality donors, and expecting FMT to get great results with low quality donors. The gut microbiome is incredibly complex, and we are so far from understanding it well enough  to be able to replace whole stool with synthetic FMT. Though I’m not discouraging people from trying it, and making headway there.
I would recommend anything by Martin Blaser. I also have a wiki section here on the permanent damage from antibiotics, that extends even beyond their killing of microbes: http://humanmicrobiome.info/Intro#more-effects-of-antibiotics
There is a tremendous amount of antibiotic overuse/abuse in the medical system. The current guidelines are likely far too generous in promoting their use, and there's even 30%+ overuse according to current guidelines.
I had an extremely depressing related event recently. I had a donor applicant that was seemingly perfect in every way. Their physical condition and ability were amazing/perfect. Their mental condition seems fantastic as well. But someone gave them frequent amounts of antibiotics over their lifetime, which was almost certainly unnecessary. And now they're suffering the consequences of it (in seemingly-subtle ways).
There are research groups that have largely given up on finding ideal microbiomes in modern society, and have thus resorted to visiting remote tribes, such as the Hadza .
I know from experience (unfortunately only temporary) that such "ideal donors" do exist in modern society. They're just very rare, and it even seems that the people more likely to qualify are less likely to apply. Many of them are blissfully excelling in life and don't need/want to be bothered by some "weird poop thing". Many of them are making millions of dollars, or on their way to make millions of dollars and don't want to jeopardize that opportunity.
HumanMicrobes.org donor criteria are listed on the Donors page. The specifics beyond those basic criteria are not listed for a variety of reasons, including, that they may change over time as we experiment and learn more, and that we need people to be honest with their applications.
Support for athletes as donors is listed here: http://humanmicrobiome.info/FMT#impact-factors
You are definitely correct that many athletes may be doing harm by pushing themselves beyond what their body is naturally capable of.
I've screened hundreds of college and professional athletes at this point, including gold medalist Olympians. The vast majority of athletes don't qualify or rank high.
There are companies trying to identify that, and use it to create "synthetic" FMT capsules, but it's largely not yet known, and in my opinion it will be decades before we can replace whole stool with synthetic FMT.
After all, you have to find a super-donor first.