are you kidding? Ryan is probably the most well-known US athlete on the running scene for people outside of the community. not only because of his ads but because the runners world people love him. Ask anyone about any other US marathon runner besides maybe meb and see if any of the hobby joggers know them, or even if people outside of the sport know him. He runs the same amount of races as most marathon runners, 2 a year with 2 (or even three) half marathons and a 10k thrown in (peachtree). the people that dont like him or think he's an air-head/nut job are the ones that dont like him because he's honest about his religion (and that in itself is marketable to the religious community).
other than the marathon guys who need the appearence fee money or arent suited for the distance the entire year, which marathon guys run track? or cross for that matter?
even if there is a "younger" generation waiting to pounce on him, hall still has plenty of running left (he's 29?) and because he's got such a fantastic resume already for a us guy he is still able to grab heft appearence fees. despite what you say, hall is still VERY marketable right now and will continue to be at least until the next olympic cycle (and thats assuming he struggles either performance or injury wise for the entire time)