This is a good post. If he is not done, I agree it would help his marathon to actually sharpen again at 10,000, then do another buildup for a fall marathon. Look at how Ritzenhein's marathon buildup under Hudson led to his epic 2009 under Salazar, the 12:56 5,000, the 6th place at the world championships 10,000, and the 1:00:00 half marathon after he sharpened from the marathon base. Frankly, Ritz would've run a much faster marathon at London with Hudson had he simply not gone out in 14:37 for the first 5k, especially with his common late race issues.
Rupp might be done though, and who could possibly blame him? But, why not run one more New York or Chicago and then one last Boston, if he's so inclined. Then call it.