I think a lot of it is a misunderstanding what it takes to run an ultramarathon and how they differ from road marathons. There are a lot of logistics in your average trail 100 miler that are not present in a road race. There is a lot more to it than just how fast you can run. From managing nutrition to gear and having a good crew. These things can make a drastic difference in time. You see people complaining about the big road marathons Americans can't run faster than 2:12 and are being beaten by 6 minutes or more, probably these same people complain about ultra's and don't realize that 6 minutes doesn't mean sh*t in an ultra. That could be the difference in someone needing to change shoes and socks at an aid station or not. This is why the arguments that Africans could just easily walk in and start dominating ultras is silly. I am not doubting that some could, but long races are an equalizer in some ways, and often times it just comes down to who can manage themselves better, not who can run the fastest.
Jim Walmsley is still learning this lesson and his desire to run shorter races now is a reflection of the difficulties of the logistics involved IMHO. Or maybe he just realizes he needs to get after it before he gets too old to run fast at the marathon.