I think you have it exactly right. Both parties actually do better with unconventional or at least new candidate. Jimmy Carter and Bill Clinton weren't new to politics but compared to the mostly Senators they were up against they were fresh faces. It was a bit easier George W Bush to make the case he was an outsider than McCain (although they both had pro's and con's in the front.)
Overall what I find absolutely shocking is the complete lack of diversity of Democratic candidates. For all the talk of diversity in the Democrat party, it is all such a superficial diversity, race, sex, sexual orientation. So many of Democratic politician went to law school, then got jobs as staffers or in the government, or academia.
Al Franken is about the last non-politician I can remember the Democrats having with some national prominence. If you think about there are probably 10 Democrats in the entertainment field for every Republican. But Republicans have had Reagan, and Arnold, Clint Eastwood and Congressman Sono Bono and a lot of sports stars. Now, I think it is ridiculous to run for President without having some political experience. But for Senate, or Governer, or big city Mayor, it is fine. You don't have to work your way up from city council member to state senator etc. Precious few Democratic politician have had much of life outside of government or law. Where are the Democrats who ran charities, invented something, directed movies, found the Titanic, or did anything in the private sector?
I think there is little chance of convincing the America public you want to fix the system if you've spent your whole life in it. Unless you have tons of charisma, like Obama or Bill Clinton.