I think this is pretty straightforward.
Bernie has a very loyal base — some Democrats, and some lefty independents eligible to vote in primaries in certain states (or register same-day.) His floor in polling and primaries is high, IMO.
Much like how Trump rode his committed supporters to victory, the longer the Democratic field stays 8+ through several primaries, Bernie’s support base + crowdfunding method will see him outlast the rest of the crowd, barring a major blunder on his/his campaign’s part.
Even Biden coming in at this point will likely not peel too many loyalists from the other candidates, and so Bernie winning Iowa and New Hampshire will likely swat Biden away.
If I were advising a strong Dem candidate like Harris, Klobuchar, or Warren — I’d be scrambling right now to lock up a strong VP candidate and convince the weaker candidates to step away from the race.
If I was Bernie, I’d be quite pleased to see Beto and whomever else wants to join and siphon off votes from my strongest competition.