I’m not sure why this has to be American-only (someone else said that, and I don’t agree)? If there’s an engaging and appropriately competitive US-based Canadian, Mexican, Brazilian, Australian, Eritrean, or, yes, Kenyan distance runner who is willing to compete for the stakes offered, I don’t know why the series wouldn’t offer them a contract. I imagine most of the contracted series runners for an American series would be American, just by virtue of geography, but that doesn’t need to be a requirement. And races already select their elite fields judiciously (or, at last, good races do, anyway)—any that aren’t willing to do so probably wouldn’t be a fit for the circuit.
As for it not attracting the top very top athletes (or even the top US athletes), agreed—top athletes would likely drop into series races when it fits their training, like they do now (Houston half as a winter checkpoint, Jacksonville before the spring track season, TC 10 Mile as a pre-New York tune-up), but not not sign a series contract or race enough times to win the series. But with appropriate incentives, while Emma Coburn, Grant Fisher and Elise Cranny aren’t walking through that door, I bet some post-peak Ben True and Molly Huddle type ”names” would if the purse were as outlined above, and a lot of Zap/Tinman/Roots/NAZ/second-tier Team Boss-caliber athletes (many of whom have good followings despite not having global or even much national hardware) would. And if it’s successful and the purse can be increased, over time you may see a higher caliber of athlete participate (outside of a major marathon or bonuses for global medals, there aren’t too many six-figure paydays out there), or it may remain successful with that tier of athletes. Not to be crass, but I think narrative would matter a lot more to the success of a circuit like this than whether Karissa Schweizer wins Wharf to Wharf in 30:50 or Emily Durgon wins it a minute slower.