While its true that some very good 10000 runners don't run the marathon well off the bat, tit is also true that Olympic golds and world records have often been taken by people who were thought of mainly as top 10000 runners not long before. Lopes, Radcliffe, Shorter (remember he was also the top American in the 10000 in 1972), Tergat, Gerbresellasie. Maybe the best Olympic examples are Viren and Zatopek... just jump in there after winning a 5/10 double, and make everyone shit themselves.
I agree that many great 10k runners make great marathoners. In most (if not all) cases those runners were successful in their first marathon as well. I don't see Rupp struggling to finish 7th, then going on to be our best marathoner in a few years. IMO, he'll either be good right away, or not at all.
Clearly, I could be wrong. There are certainly cases of modest runners starting out poorly then running far better later on (Luke P is a recent case). Those are probably the inconsistent runners in the first place. Rupp does not seem to be that type.
However, I can see him misjudging the distance and blowing up. Then doing much better the second time out, but the OP suggest that it will be a slow process to Rupp eventually being a top marathoners. My gut (and experience) tells me that won't be the case. He'll know after the first one if it's for him.
Time will tell.