I don't understand how so many people who are supposedly political observers seem to cling to this idea that he won't eventually endorse the Democratic nominee...
Well, Eugene McCarthy, Ted Kennedy, and Jerry Brown didn't endorse the democratic nominee. It's not unheard of for a democratic primary to end in bitterness with the loser making unreasonable demands and refusing to endorse the winner.
The thing is, people remember McCarthy and Brown as tremendous assholes for what they did. Nobody today remembers Jerry Brown's 1992 campaign fondly, he's remembered as bitter, petulant, and caught up in a one-sided feud with Clinton that Clinton couldn't care less about. Eugene McCarthy's 1968 temper tantrum is the case study everyone points to for how someone could completely screw up a convention and presidential race, and he had far more of a case than Bernie given that the race was in complete flux after the assassination of Bobby Kennedy. The only reason Ted Kennedy isn't viewed as negatively is because most people are aware that Carter was a dick to Kennedy throughout his presidency.
What is Bernie's justification for going to the convention? Clinton has been extraordinarily kind and gentle with him throughout the primary process, not running a single attack ad against him. His claims of the party stealing the nomination from him and being unfair to him or not respecting his supporters are just a bunch of insubstantial fluff and he knows it. McCarthy and Kennedy at least had motivations for refusing to endorse. What is Bernie's motivation? Is he just Jerry Brown '92 redux? That's the most irritating thing about his issuing of demands and his insistence on going through to the convention. He doesn't seem to have any actual good reason to do it other than that he wants attention (a.k.a. "the movement must survive", "the revolution must continue", whatever) or, as the Politico article inside his campaign revealed, that he's just bitter and angry at Hillary Clinton and the DNC in a one-sided way.
Good, insightful analysis about McCarthy, Kennedy, and Brown. Of course, all three ran under very different circumstances. Brown will complete a comeback and a largely positive legacy as a 16 year governor of the largest state in the country. And Kennedy is rightfully regarded as a beloved Democratic figure.
On the other hand, despite his views on the Vietnam conflict--McCarthy today is regarded mostly as a bitter, dysthymic malcontent who was in love with himself. McCarthy thought that he was bigger than the cause. Bernie can do the same--or he can work within a system to make gradual changes.