On the Hadrian Wall itself, we may well profit from the expertise of AC, Caldrail and other UNRV members regarding when, why and by whom was the inner moat constructed and filled, as well as the potential interaction of tactical and logistical factors behind such events.
Anyhow, regarding the original question, that's inconsequential.
No human institution is perfect; I'm positively sure we would be able to find many undisputable stupidities done by Rome and any other successful empire during either their high or low periods. So?
The raw idea (hypothesis?) that Hadrian Wall's inner moat construction (or destruction?) was a sign of utter (and new?) stupidity (???) to such degree (and timing?) that it explained in any meaningful proportion why the Roman Empire was unable to defend its borders (is that what Mr Shenkman understands as Rome's fall?) simply doesn't stand by itself as it was presented in the first post of this thread; too many unexplained facts to be even falsifiable.