I've worked with lots of runners. So far every single one of them that has committed to running 40+ mpw year round, done workouts properly, and raced with conviction has gone on to break 20. The fact that I have never seen someone fail to do it certainly makes it a little hard to buy into this idea that most cannot. Certainly there is a level where that becomes true, but I'm just not seeing 20 minutes as that level.
Now, I realize all those things are asking alot, and that this is a very small data point in the grand scheme of things. However, we are talking about having the potential to do so.
I'm also not 100% sold on this idea that people that don't come out for XC are the unathletic ones. The only way to even know if they are unathletic is to have tried sports, and just because you are bad at basketball or football doesn't mean you are going to be poor at running. You can have the qualities needed for being a good distance runner without being a competent ball player or football player.
Another interesting analysis is that comparing it to the sprints, based on percentages, is the equivalent of saying most guys couldn't break 15 in the 100m. I've seen the rare exception, but most men, in shape, have the intrinsic ability to run 15-16 mph, even without training. I could run 13 high/14 low and was definitely one of the slower kids growing up.