Aside from all his racial gaffes, the heavy baggage of all of his previous policy positions that don't look so good anymore, his inability to speak coherently off the cuff, the fact that he does not appear to be aging well and if elected would be the oldest president ever, and his completely unrealistic view of what the modern Republican party is I can't think of any other reason to not back this guy for President

Oh wait, here's one more!

Incumbency is a huge advantage in any election, including the Presidential election. Most Presidents have won a second term. Of the 9 Presidents in my lifetime only two, Carter and Bush the First, didn't win after running a second time (Ford was running his first time). Why would the Democrats ever give up that advantage with their Presidential nominee?

Also, Biden is presumably only saying this because he thinks it will people a reason to support him. But what kind of sales pitch is "support me and you'll get less of me"?!?!?!