Boston isn't a PR course, its an unlaced championship style race, who really cares if he ran a safe race and got 7th or 8th in 2:08-2:09. Mantz and Albertson raced to win win and WENT for it. It's a good thing for the sport in the US when the Americans actually try to win and are in the lead pack for 75% of the race. Mantz went for it, even when he was well out, he saw kipchoge coming back to him and spent everything to try to beat the goat before imploding.
Mantz could have been the top American if he wanted to be. But he wanted to compete to be the best runner on the day, not get a silly asterisk title. He knew the risk he was taking and paid the price for it.
Having Americans on TV for the majority of the race is good for the sport. The average person stopped paying attention before Fauble finished, but they will remember Mantz and CJ Albertson battling for the win. Plus sometimes it might work work (ex. Molly Seidel in the olympics). I would much rather have Mantz and Albertson on the Olympic team who will try to medal than have Fauble who will run a boring conservative race for 8th instead of giving an honest attempt at a medal.
People on this board constantly complain about people time trialing to much and not racing. We get one of the more promising American marathoning prospects who is willing to send it and commit to staying with the leaders while they were on WR pace through 4 or 5 miles, and suddenly people are complaining he didn't pace well.
TLDR: Mantz gave everything and competed with the best in the world, that's not a disappointment.