How exactly is an old white man who has spent decades as a Washington insider going to appeal to disaffected voters?
Let alone when you toss in the fact that he's a socialist, Sanders would be screwed either way.
It's all about policy. Bernie talks of radical change, Bush talks of the status quo. That type of reformist rhetoric is appealing.
But, you're right, Bernie would immediately be on the defensive. Obviously. However, the debates would be to his advantage. He is a very good debater. And so, he could present his policies in a down-to-earth manner as opposed to the red baiting in Bush's attack ads.
All I'm saying is that campaigns are unpredictable, complex things. Who knows what could happen during the campaign. It's anybody's guess.