If Trump is to be defeated in 2020, it will have to be (A) because he doesn't know what he's doing and hasn't gotten a grip on things, and (B) the Democrats have nominated a candidate who's ready to be a REAL President from Day One. To beat Trump, "The Joke's Over!" has to be the theme of the 2020 campaign, and that assumes that events will have set Trump up for such a scenario.
In that scenario, will Cory Booker really have enough experience to make it to the Presidency? He's a first term Senator who was mayor of a heavily black city that was not attractive to business, and whose urban problems are far from insignificant. I would suggest that in such a scenario, folks wouldn't be turned off to a candidate with DECADES of experience. I would think there are limits to the "outsider" thing.
I don't think that would be much of a problem; a several term mayor followed by 7 years in the Senate is about as much time or maybe even a little more than any governors would have served. While I agree that depending on the electorate's mood someone with a LOT of experience could be called for, if you don't assume that that in advance I don't think the experience argument would work very well.
