A much better comparison would be to an MMA event, rather than football. As we all know, if Tom Brady is at 80%, he's still way better than any backup (and many team's starters), and his team can gameplan around that since there are 10 other people on offense (plus defense and special teams) to share the load. If Centro races that mile at 80%, he gets DFL and embarrassed, just like if a fighter goes out there at 80% he gets knocked or tapped out. As a result, we see tons more changes to MMA cards due to injuries/illness compared to team sports, where injuries are as common (or more common in the case of football), but playing through injuries is par for the course.
Now certainly if whoever is headlining an MMA card drops out, it hurts the card, and can quite a bit in the case of the biggest stars. But within that sport's world, the top brands have been successful in marketing their entire roster, rather than just 1 or 2 stars, and the talent is generally there that things are still competitive if a #1 contender drops out and they fill in with a #5 ranked person last minute. Of course it doesn't hurt that if someone gets completely manhandled it still makes a compelling highlight reel shot, where there is no such luster in T&F.
Now I didn't watch the race, as I don't have any intentions of paying money for what is so consistently an unwatchable production, but I'd be willing to bet they didn't have more than one or two lines to share about Murphy or Blankenship, and made almost no attempt to 'introduce' us to any of the gaggle of Kenyans outside of Kiprop and maybe Kwemoi, I'm also betting the whole broadcast was on eggshells about the current NOP/USADA situation, which does nothing besides allow the issues to fester (regardless of whether you think NOP is guilty of doping or not). NBC should have had Centro in the stands at the finish line, or try to get him in the booth for NOP PR and to explain the illness and make commentary on the race.
That's how you build up more following for the sport among the fans who are there for 1 month every 4 years, by building up and marketing a critical mass of competitive athletes, not putting all your eggs into a handful of athletes who compete 4-6 meaningful times per year, with a 25% chance of no-showing any one of those.
All that of course has to come after they fix their basic incompetence on the broadcast, which has been beaten to death dozens of times here.