I think it's going to be very difficult for an American to win a World Marathon Major, even Shalane.
1. The talent pool on the world stage is so deep that even when one of the top Americans races, they are one of 8-10 runners with a shot, giving them a low percentage shot of winning to start with.
2. Within that 8-10, there are usually at least a few other athletes with better credentials, lowering the Americans' chances further.
3. Americans with a shot to win are usually built up as contenders in the American media, and therefore try to race with an Alpha Dog mentality (see Ryan Hall taking it out hard in Boston, etc, vs the races earlier in his career when he laid back a bit more...it's not a huge difference, but every bit counts at that level). In these races, it takes a supremely fit (and usually superior to the other athletes) runner to win running that way, and no Americans seem to be at that level.
When Meb won NY, it was the perfect storm. He was unheralded for that particular race, was able to run the race in a way that saw the other contenders destroy each other so they were toast when Meb finally went to the lead. He was extremely fit, had a great day, and nobody else did.
With all that said, I think Shalane has about as good a shot as we're going to see an American have in the next handful of years. I would be mildly surprised, but not shocked.