Ah, well here is the problem. Suppose a candidate like Biden or someone else is nominee, and this person was considered the best choice by many "experts". But suppose they just don't arouse a ton of enthusiasm. This could easily lead to many dems either staying home or defecting for third party candidates. If Biden makes the mistake of changing positions a lot, he'll compound the problem, as it will lead many independent voters to see him as ingenuine or a "flip-flopper". He needs to watch his ass and what he says if he is the nominee.
That's pretty much what happened in 2004. And it happened in 2016. A lot of former Bernie supporters did vote for Clinton, but while they might have been very enthusiastic and encouraged others to vote for Sanders, what you had were a lot of disappointed people who just did the bare minimum and pulled the lever for Hillary, but otherwise did nothing. Not to mention Sanders voters who either stayed home or voted for Stein or some other person. I remember an NPR story from 2016, right before the election, and they spoke to former Sanders supporters who were voting for Hillary. Some said that had he been nominee they would have been actively campaigning, going door-to-door and encouraging others to vote for him. But they said they just didn't have it in them to do the same for Hillary, especially after all of the DNC shenanigans the previous Spring and Summer.
Biden is not a candidate who's ever really aroused a ton of enthusiasm. He was a great veep, but look at all his failed runs for prez. People need to be careful not to assume Obama's popularity will carry over to Biden. Gore assumed he could just ride into the white house on Clinton's high approval ratings.