Your post is totally flawed logically and also is factually incorrect.
1) Factually incorrect: Ben True has a 3:36 1500 pb. Bernard Lagat has run 3:26/27:49. Maybe you don't consider them to be 'great' American 10k runners but you get my point.
2) Logically flawed: The biggest problem with your logic is their are way more marathon qualifiers than there are track qualifiers. There will be more than 150 Olympic Trials marathon qualifier when its all said and done. How many make it in the 10k? 30? So if you go down to the 150th time on the track for the 1500, then a lot more 10,000 runners could do it.
Look, Walmsley is a pretty darn good flat runner. I doubt there are that many Americans that could beat him but in terms of the world, yes.
On our podcast yesterday, I made the argument that I thought a 2:05 marathon guy could crush Western States. I said something along the lines of I'd love to get a 2;05 marathon guy in top marathon shape and pay him to sit on Walmsley at Western States. Initially, I said I thought he could do it without training specifically for it but as the podcast went on I talked off air with John Kellogg and he said they'd have to train for it.
But the idea that Jim Walmsley would be the best in the world at the ultra if everyone in the world competed at it doesn't make much logical sense to me. As a former math guy, the odds of that just don't work for me. Walmsley is like the equivalent of the best runner in a single state - say Michigan. What are the odds that he's also the best runner in the US if he shows up at nationals. Not very good.
African runners dominate world xc and they dominate the marathon. If you combine the two, why wouldn't they dominate that? Now I"m totally into Sports Gene so maybe David Epstein or someone could explain to me they dont' have that fat stores or muscle strength, etc.
I do think it's interesting as we don't know if it would happen. And it looks like different types of people win Comrades/Western States, etc. Is there any ultra person that can win everything?
I have no doubt about it if it was a 100 mile race on a road. Some trail thing leaves a lot more doubt.
Regardless, Chris Chavez played my comments to Walmsley . When he heard them, he got excited and said he'd LOVEfor a 2:05 Ethiopian to try to sit on him. Go to the 56:00 minute mark or their podcast that just went up.
Here is what Walmsley said after hearing me say I'd love to take a 2:05 Ethiopian and have him sit on him at Western States (They didn't play the clip where later I said I spoke to John Kellogg and he'd have to train for it),
Now I'm sure in this day and age of faux outrage people will try to promote some conflict between me and Walmsley. I actually loved listening to him talk, he was very thoughtful and I actually think we pretty much are in agreement. I underestimated how they'd have to train for it (But Kellogg convinced me in my 20 second talk to him while I muted my podcast) but my argument that someone else would be the best in the world is still one I hold to and one that Walmsley seems to agree with.
(On a similar note, about 8 years ago, I once asked John Kellogg if I got Weldon on drugs could he easily make the Olympic team. He laughed and said it would take a year for Weldon to be able to get his legs back and train enough (100 mpw) to make the Olympic team).
Here is our podcast.
http://www.letsrun.com/news/2019/01/dubai-houston-nbigp-letsrun-2019-podcast-marathon/Here is Citius Podcast (go to 56 minute mark)
http://citiusmag.com/citius-mag-podcast-jim-walmsley-houston-half-marathon/