I think there's some truth to both of these positions, but surely neither holds for every voter. If we're assessing his net impact on American politics, the question is whether those considerations outweigh any positive contributions he's made to building the left; we have to do a cost-benefit analysis, rather than simply pointing at costs and concluding that they're dispositive. And some of those benefits, the Washington Post suggests, are extraordinary:
That latter point is crucial, because one's early experiences don't just ratchet in a party preference - they ratchet in a whole way of thinking about and relating to politics. Right now, what young people all around America are learning is that social democratic policies are not just preferable, but plausible, and in fact almost within reach; that "socialism" is not a word they need to run away from; that liberal Democrats are not only unreliable allies, but often a major obstacle to their aspirations; and that left political campaigns are uphill battles for ideals that terrible people will dismiss as radical and unrealistic. A substantial body of evidence suggests that they will carry these lessons with them for the rest of their lives."...He's moving a generation to the left," Della Volpe said of the senator from Vermont. "Whether or not he's winning or losing, it's really that he's impacting the way in which a generation — the largest generation in the history of America — thinks about politics."[...]Della Volpe cautions that it's impossible to predict how millennials' views will shift in the future, but people change parties only rarely after about age 30, researchers have found.
Compare that to the alternative: if Sanders had not run, there are plenty of young people who would have learned the exact opposite lessons. We have some significant data on this. As of October 16 of last year, here are who Sanders voters were calling their second choice:
These numbers would likely be somewhat different drilled down to voters under 30, but other indicators suggest that faction of young voters who might have considered another candidate is substantial. For example, YouGov / Economist suggests that Sanders got a 10-point bump among young voters between October 12 (36%) and November 9 (46%). That just happens to span the month when Joe Biden dropped out of the race; Biden, incidentally, was winning 12% of the Millennial vote.
Instead of supporting any of these relatively centrist candidates, advocating their neoliberal policies, and rationalizing their establishmentarian affiliation, these young Sanders supporters have spent their formative first year in politics arguing against such voters. This possibility was always latent in their politics, but it needed the right candidate to catalyze it, to give young people a viable alternative that they could rally around. There is no reason to believe this would have happened without Sanders, and every reason to believe that this impact on American politics will resonate for generations to come.