In the abstract, there are quite a few decent candidates that could run on the D side. Whitmer and Shapiro from PA are the cream of the crop if you ask me.
It's the "how do you get there from here" question that kinda derails the whole thought exercise. The reality is that if you do not want Biden to run, you have to come to terms with Kamala Harris being the nominee. And unsurprisingly, many of the same people (at least in media) who have complained about Biden running the most (Bill Kristol, Nate Silver, others come to mind), would ***** and moan about her being the nominee as well.
I'm not saying he should run at this point, I've been concerned about his age and running for reelection for a while.... but lets not pretend that they could easily and bloodlessly meander their way to Candidate Whitmer.... that's a fairy tale.