Biden isn't even running, may not run but he consistently polls first among possible candidates and gets almost twice the polling numbers that Bernie gets. Bernie has never gotten out of the teens in polling against a field of Democratic party candidates. If Biden does not run, it's doubtful those Biden voters will pick Bernie. Bernie spent his time in 2017 sowing division, did not build bridges to coalitions that did not vote for him in 2016 and continued to alienate Black voters the south throughout last year. He is good in his role as a Cassandra in the Senate. So, don't say I never said anything good about him.
How Bernie Sanders Could Win The 2020 Democratic Nomination
Sanders enters the 2020 race not as an underdog but as a Democratic Socialist leader of the pack; an early Iowa poll showed him commanding 19 percent of the vote of likely caucus goers, second only to former Vice President Joe Biden. Sanders comes to the race with the high name recognition that many candidates in the crowded field lack, and with a glossy pelt hanging off his political belt: the grassroots movement that propelled him to unexpected heights in 2016.
https://fivethirtyeight.com/features/bernie-sanders-2020-democratic-nomination-kickoff/
The article says "could win" but spends most of it's time listing why he won't.
Plus side: Enters this cycle with high name recognition, grassroots movement remains from 2016, a plurality of voters in midwest rust belt states like him, young voters like him, socialist policies are better accepted by Democrats compared to earlier times.
Negatives: Poor image from losing in 2016, voters see him as un-electable in the general election, problems with black voters, problems with women voters, message is not unique in 2020, viewed as too far left by moderates, not well like by older voters, Bernie Bros are odious.