this thread is dead, but why let a zombie rest in peace...
Apples and oranges.
O.P. tries to manufacture a zero-sum game out of two fantastic performers who are not natural rivals, in age or in talent bias. They are both great. Lagat is a class act and his career will indeed be legendary, based in large part on its longevity, assuming it ends without mishap. Legends are rare, but Rupp is already an unprecedented historical figure in running and has already outperformed Lagat (at any age) in anything over 5K. Explicit references to one of them being a groin-level servant to the other is indeed the essence of Trolling.
Lagat focused on the 5K and won. Rupp focused on the 10K and won. It's a win-win for U.S.
Galen at 38? WTF. Almost no one at 38 has ever done what Lagat does. Lagat's ability to maintain his high-end gear into his late 30s is preternatural. Jealous trolls speculate on Rupp's natural vs enhanced "Franken" abilities, while never questioning a much more remarkable ability to hold and even elevate such performance without serious setback through a second decade. The ageless Pettite, Clemens, Bonds, Armstrong, Jeannie Longo, MDSlaney, ringing bells anyone? To be clear, there is ZERO evidence either Kip or Galen has abused their gifts, but Galen's achievements are not the most astounding. If Lagat is already judged a legend and Rupp never is, fine. But if being "A Legend" is a Requirement to avoid carrying another's nether garments, i'm sure we all know what SPMB's occupation is ITRW.
Rupp's Never been great at 1500/mile so the premise that he'd not hold a candle at 38 is trivial. Lagat at 38 focused all effort on one event, a 5K Champs neg splitter exacerbated by heat that became a jog followed by a 3:40 time trial for Lagat's (and not Rupp's) specialty. Lagat ran smart, he ran strong, he showed class, and he'll need much more in Moscow for any momentos. Rupp ran for placement and he ran smart in taking over at 400 and kicking from 300. I'm sure he wanted to win, but it was never likely in the cards given the heat, the 10K double, and pre-peaking form, so when he came off the corner with Lagat pulling around him, he was only concerned with holding his qualification, not straining himself to retake Lagat like he did last time - watch his head flicks.
The OPs hypothetical left out the other part of the equivalence. Rupp at 10K. Lagat has NEVER demonstrated worldclass talent above 5K. Although his 5K PB is 5 sec ahead of Rupp, his 1/2M is 2 min behind (and Rupp fell mid-run). If Lagat had accepted the increased base volume needed for strength at 10K, would he have remained injury-free and effective to 38? Did he have the talent to have ever successfully qualified at 5K+10K, against current U.S. competition level? Double US champ? Rupp could never do what Lagat does, and Lagat can't now and probably never could do what Rupp does. But i can't see how there's a negative in there. It's two positives.