I don’t think this would water down fields very much. Unlike other marathons, only 3 people are going to be entered per country. Ethiopia and Kenya will be almost unphased, as will Japan id bet. The US’s best athletes normally only race Boston/Chicago/NY anyway, and even then don’t make up most of the top 10.
I don’t like that they do it every other year for each gender. I guess that’s to help prevent watering down the fields, but what’s the point of having it during Olympic years? Unless there will be a track WC during Olympic years, it doesn’t make sense to me. I guess maybe it’s good for marketing?