This has been proven true more than once. Whenever "their turn" candidates run, they loose. See Kerry, McCain, Romney. Socioeconomics aside, Americans love novelty. With Trump in power, the game has been elevated to a different level and I think none of leading Dem politicians can unseat him in 2020. After a sufficient time has passed, when his novelty has worn off, some red blooded alpha-male type has to burst into scene and start challenging him. I am looking at you, Jason Kander!