Zat0pek,
I think part of this American stigma about staying at 'your distance' and not giving higher distances a chance is somewhat linked to the idea of making concessions to failure. I think a lot of people view moving up in distance as admitting failure to do well at the distance they have chosen to try excelling at, and some athletes are the athletes they are because they always want to hold out on hope. Can't blame them, because hope is a good thing.
Now I'm not justifying this point of view; just saying that I could see a lot of top athletes out there having this perspective somewhat embedded in their thought processes. Especially if they have spent so much time training for a specific distance, and maybe that's the error to begin with. Canova believes strongly any training the body more aerobically until the athlete is in his early to mid 20's, and then working at specific distances. If you look at some of his athletes, they have competes at various distances: Isaac Songok ran 3:31, 12:48, and even some longer cross races. Kipchoge has started his seasons out the past couple years with 10k's, both under 27:00.
One example I think of is Anthony Famiglietti. For years he continued to pound away at the steeple. Then in the last few years, we've seen him run 3:35, 7:39, 8:13, 13:11, and 27:38. He set ? PR's in a two-year period just from racing different distances, and how many 10k's on the track had Famiglietti ran before his 27:38? I see your point, and I think it has some validity to it, if not a lot.
I believe the crucial point in your argument is that if a runner has a limiting athletic ability (i.e. their basic, raw speed is only so fast) then they can improve their caliber of competitiveness by adjusting the other parameters of their physical characterstics which are not yet limited. I think there are some runners in the U.S. who can benefit from this mindset, but at the same time, I think there are a lot of athletes who see competing at distance of 5-10k as futile against the top runners in the world.
For the 10k, a man really has to be able to run under 27:00 if he hopes to be competitive in the least against the world's top talent. Until some U.S. runners start running within spitting distance of 27:00, middle-distance runners will not stop perceiving the 10k as a distance out of the American league. I think Hall and Rupp are two candidates possibly capable of approaching to within spitting distance of the 27:00 'barrier', but until some runners start doing it, no one is going to start looking for the carrot at the end of that stick. Like a poster said earlier, the carrot looks a lot closer when it's only 3.5 seconds away as opposed to a whole minute. And no matter how good your basic speed is, you need a lot more to be able to run 26:30 for 10k. It isn't as simple as just logging more junk miles.
I think there are some special cases as well. For example, Webb has amazing range, and I think he's make some good decisions, but I also think he has a great opportunity to lower the 3k and 5k AR's. Anyone who can go 1:43, 3:30.5, 3:46.9, 8:11, 13:10 and 27:34 in a debut has very good potential for any specific mid-distance to distance work. Remember, Webb's 5k progression went 13:45, 13:30, 13:10. And his 13:10 was ran at the end of the track season, not necessarily when he was at maximum strength or even specifically training for it.
I often think about Seb Coe. He was already talented at 800m when he was in college, and his 800m speed is most definitely the reasons he was able to get gold in the 1500m, even though he was the WR at 800m. At one point, he lost that pure speed, but still had enough of it and stamina to be great at 1500m. But Coe never really moved up in distance, and there is an interview where Peter Coe mentions that the time came when they could have started moving Coe up in distance but instead decided to turn around and keep focusing on speed. But I imagine Coe could've been a great 3k-10k runner had he spent the time training specifically for any of those distances.
I think Webb should definitely make a coordinated effort to train for the 5000m if he still wishes to continue running 1500m/mile. I could see Tegenkamp being good at 10k if he can hang with the leaders, but I don't think he has the aerobic power to do it. He definitely has the finishing speed, but if he can't be able to withstand 60 second laps being thrown in the middle of the race and continuing on for another 10 laps then I'm not sure if he could be any more competitive at 10k on the world stage than he could be at 5k.
Webb --> should try 5k's and 10k's more, a la Kipchoge
Tegenkamp --> should try more 10k's
C. Smith --> 1500's and 3k's
G. Jennings --> marathon
Sara Hall --> 10k
Tollefson --> marathon
Rupp --> turn pro and start marathoning but still do 10k's
McDougal --> marathon
Bauhs --> marathon
Manzano --> 3k's/5k's (at least try them)