Copernicus said:
Bernie will certainly be doted on by the media, but his time has passed. He never really did find enough traction in the Democratic Party, partly because he has always held the party at arms-length. He joined the Democratic Party only so that he could run as a viable candidate. Shortly after the 2016 election, he quit. Now he comes back, hat in hand, asking for a second chance to be a Democrat. He still has enormous influence with some parts of the base, although his support for anti-establishment Democratic candidates has had mixed success. But he will also find a lot more resistance to his candidacy this time around, as former Clinton supporters recall his stubborn attacks long after it was clear that he had lost the nomination. OTOH, Sanders has always loved to run as the outsider, so he isn't going to fade away gracefully.
The above is basically the reason everybody should support Bernie. A political party should exist to deliver policies that its constituents want, and Bernie is the champion of all those policies. Policies should not be tweaked to serve the interests of an established political party. That he is an outsider to the Democratic party, but represents the will of most people who consider themselves Democratic voters,
is a sign that the party is in the wrong place, not that Bernie is in the wrong place. Voters who consider themselves far closer to being Democrats than Republicans or even 'independents' overwhelmingly support the ideals and positions that Bernie has unapologetically, uncompromisingly held for decades. Every other candidate, with no exceptions, has drifted to a lukewarm compromise on these topics (thanks in no small part to pressure from voters galvanized by Bernie in 2016), having held contradictory positions on them in the recent past. It's pure political opportunism, which is what the Democratic party has always stood for. And Bernie is the only way to make a dent in that, even though he falls short in many ways of being a perfect candidate.
I think that you, like many Americans, have a problem accepting the nature of our two-party system, which was something that evolved in American politics shortly after the Constitution was ratified. Political parties in a two-party system are necessarily coalitions of special interest groups with largely (but not necessarily) compatible goals. Hence, the Democratic Party includes constituencies that seek to improve minority rights, but Bernie Sanders was never really seen as someone who was deeply invested in those issues. He made speeches and attended rallies in favor of those issues, but he wasn't seen as someone who could inspire the African American or Latino voting base. Sanders had his greatest appeal among young, white, and middle class Democrats. Hence, Hillary Clinton was usually able to beat Sanders in primaries where a more general cross section of Democrats would show up, as opposed to caucuses, which attracted more ideological party members. The most successful Democratic candidate will be that one that is best able to bring together the diverse elements of the Democratic coalition.
Bernie Sanders has great appeal to a base of loyal supporters, but one thing he is not is a coalition builder. His only role these days seems to be in dividing the party. That's why Republicans are so happy to see him jumping into the race. Most people realize that his support isn't going to snowball in the way it did in 2016, when he started out by pledging to conduct an issue-oriented campaign that avoided personal attacks but ended up with supporters vilifying the party nominee long after she was clearly going to be the nominee. He is too old now and far less popular than he used to be.
See
How Bernie Sanders Could Win The 2020 Democratic Nomination for an insightful analysis of Bernie Sanders' chances in the 2020 primary season.