The networks show what ratings prove is watched the most, they have genuine measures for figuring this out. Joe Sixpack (even John Marathonfinisher) is not clamoring to see Virgin London, BoA Chicago, BMW Berlin on tv, either. That type would likely not even DVR those should they be available via free broadcast -- unless, of course, Hall or Flanagan were in the race, but I digress. Like Ryan pointed out, too much competition for eyeballs in the U.S. media market now. Baseball and softball were removed from the Olympic program despite the U.S. teams being dominant medal contenders in each, primarily because it turned out to be just the same few nations contending for medals each time and it became uninteresting to watch, even in the country that invented the sport(s) and where the sport(s) remain significantly popular on all other levels! In our sports-happy and baseball-loving country, the general sports fan viewership would rather tune-in for preseason NFL than for Olympic baseball and softball, despite the usual promise of seeing the home team get a medal. Other nations stopped assembling and sending teams for baseball and softball because they did not see the merit in simply showing up and 'fighting well' only to get clobbered and see the other nations on the medal podium time after time. The marathons may not be removed from the Olympic program once they become a de facto Kenyan championship and thus uninteresting to the general Olympic broadcast viewer (though it would not surprise me if they were), they might simply get dropped from the broadcast stream to the U.S., U.K., Europe, etc. because advertisers do not want to pay for airtime that consumers do not tune-in to. I feel that would hurt the sport domestically.
I think you may have missed something about Burfoot's proposal, however. It would not be only the top 30 performers to go, it would be the top 30 in addition to the standard 3-per country (more or less) for all others. The issue with that could easily be what we saw happen with World Cross in that the best runners from a given (non-African) nation will skip it and we might see U.S., U.K., Japanese, Australian, etc. teams consisting of second or third tier (in their countries) athletes who can get under the Oly A standard but have little chance at even the top ten, would just be fodder for filling out the field -- Olympic tourists, to borrow a term. Similar to the teams we see fielded for WC marathons. Waning interest to make the U.S. Olympic team in the marathon among top runners would likely hurt the sport domestically, as well. With no appearance fees involved, why would any non-African pro runner outside of the top 15 want to go spend one of their hardest marathon efforts to run a race for free and a monumental challenge just to make the top 10, to say nothing of contending for a medal? Under Burfoot's proposal, the odds to make the podium and medal that U.S. runners like Keflezighi and Kastor had would be made essentially nil. Or would it just lead to top guys and gals taking team spots, taking the trip to the Olympics, and then DNFing by mile 20 when the race outcome for medals (or top 10) is clear so they can regroup and collect an appearance fee for Chicago, Berlin, NYC, or Fukuoka? And with no shot at an Olympic medal, it just makes a career in the marathon for a U.S. runner (as with Kastor and Keflezighi) far tougher. Would Keflezighi and Kastor (let alone runners further down the competitive scale like Hall, Goucher, Sell, Russell, Lewy-Boulet, et al) have specialized primarily in the marathon and kept after it without one marathon every year that had a limited field of top Africans and a pretty big payoff? Take away the incentive of the possibility of contending for a medal and the desire to develop to the level of a medal contender in the marathon is eroded. The only way to make Burfoot's idea work well is to add significant prize money going about 20-deep, to incentivize it for those outside the top 20 -- even for those outside the top 10, for that matter.