That was actually the 'conventional wisdom' until late October, 1980.
I'd rethink your pessimism, if I were you.
(And don't say Santorum is no Reagan. That's not the point. Most people didn't know that Reagan was Reagan in 1980, so it wasn't a factor).
Your post has excellent points all around. The arguments that Santorum is too much this and not enough that to beat Obummer is silly. He has the best favorables of all the candidates and he has a solid message. He can beat Obummer.