If you're building the new Eugene stadium and it's going to be ready by 2020, you'd rather have the Trials in 2020 and the Worlds in 2021 than the USATF champs in 2021 and the Worlds in 2021. If Eugene isn't ready by 2020, then this will turn into a really bad decision because Mt. Sac is on schedule according to their people.
I think a lot of athletes would disagree with you on that.
Most athletes seemed to be pretty strongly in favor of the 2020 Trials at Mt SAC, mainly because they are tired of going to Eugene, but I think some also recognize that with the 2028 Olympics coming, it is good to try and build a fan base in the LA area.
Eugene's original plan would have had the stadium mostly finished by 2020. When they did not win the Trials, things were pushed back. So yes, they say they can have a functional stadium in time for 2020, but it will be LESS ready and less finished that it would have been had they won the original bid.
As far as 2021 USAs, I guarantee most athletes will want to be on the World Champs track, especially since it won't be totally done in 2020 even IF they are somehow awarded the Olympic Trials.
It is a big advantage as an athlete to get to compete on the World Champs track before worlds. That's why they held USA Indoors so close to Worlds when we hosted World Indoors... the track would not be ready a week earlier.
Even though most athletes are familiar with Eugene today, the flow of things will be totally different in 2021. And even if they host Trials in 2020, it will change between 2020 and 2021. You want to get that experience under your belt coming in to Worlds.