For me the main problem is that it's a "small pond" sport. Maybe there are dozens of world elite marathoners that could beat Walmsley at the WS 100. We don't know and never will. You guys are raving about a guy that might only be 20th in the world or worse, if the best marathoners wanted to compete in trail racing.
From competing in school I know exactly how fast the paces for the 5, 10 and the marathon are and 4:03, 4:13 and 4:38 are mind boggling. Walmsley pace for the WS 100 is undoubtly fast considering the distance, terrain and changes in elevation, but it's still crawling.
Trail racing is not a spectator sport. You really can't watch in person and it will never be on TV. I went up to Foresthill one year to watch some of the runners go through during the WS 100. They ate some food, were weighed and then continued their journey. It was not a riveting sports viewing experience for me.
I don't make fun of ultra runners, but it's not a sport that interests me.