I believe the answer is a combination of depth and specificity of training. I do not think genetics is a significant factor.
Until the US has a couple dozen guys capable of at least a sub 2:10, breaking a top level time is unlikely to happen. If you paid attention to all the top Kenyans, you would see that it is rare for any single Kenyan to have that many good marathons. But there are many of them and as soon as one guy flames out, there is another superstar ready to emerge on the scene.
Ritz has had one fairly good marathon and a few decent ones. While it is possible that he will run a great one, the odds favor him never running another PR.
Now, follow this train of logic: First, lets presume Ritz had/has the potential in his career to run 2:04. He started off good with the right base in high school and college. When he hit his prime years to run a great time, he spent several years injured. Since then he has rebuilt his fitness and is in his best shape, but he missed the opportunity to reach the high peak while he was sidelined with injuries. This happens to many Kenyans too.
If we had 10 Dathans, the odds would favor at least one of those guys making it through the peak years without injuries and getting a great performance. Hall did it, and I don't think he is as potentially good as Dathan, but he was not injured during the important years.
The second point is specificity of training. The best potential guys spend a good portion of their career running track events before getting to the marathon.
I think that we have a future in the marathon if we just keep producing a ton of good runners in high school. The 800, 1500, and 5000 are already extremely competitive in the US. Once the steeplechase and 10000 are filled out, some of these talented guys (lets say 13:15 - 13:20 guys) will forego training for the track, knowing that they'll be perpetual no-names in the USA and focus on the marathon. If you carry it out to its logical conclusion, if we can produce enough sub 13:30 5000m runners, eventually a large quantity of people will start focusing solely on the marathon and a few guys will get it right. You can already see an example of this idea from Jager moving to the steeplechase and taking the US from being a complete non-factor in the event, to having a darkhorse favorite. If there were a couple other Jagers, they could run like a team and we'd be pulling at least a minor medal in the big championships. A very small change in one guy switching events has made a big difference. Regarding the steeple, if David Torrence, Will Lear, a healthy Solinsky, and a couple other 3:35 type guys switched to the event, it would totally change the steeplechase scene in the US. The marathon needs a similar congregation of high talent for the US to move forward. Too few of the very best guys are doing it. The B level guys (Arcinaga type) are running very very well so the training methodology surely works. Imagine a few guys with much better 5000/10000 credentials making the same commitment to the marathon...
Look, if an aged Meb can win the NYC marathon past his prime, get 4th in the Olympics, and still run 2:08s at his age, younger guys with his talent (or better) who are in the prime of their career should be running 2:05 at least. I think once somebody does it, there will be a flood of others.