On the whole I think this was one of the biggest reasons he lost. It allowed his opponents to just hammer away at the, "...the math doesn't work," argument every chance they got. Afaik there was no math to work, so it made it impossible to favorably rebut it. Had he had a concrete plan where the math worked even only under the best circumstances it would have come across much better than a mythical plan that may or may not work.
Other stuff probably contributed, but I think this specifically sucked out any momentum he might have had going into the home stretch.
No plan will work for a party that doesn't want to pay any taxes, more military spending than the rest of the world combined and for all their rants about entitlement aren't actually willing to give up their social security or medicare.