A Trump-Sanders race would have been more policy-oriented and less character-oriented than a Trump-Biden race will be.
Sanders’s great general-election vulnerabilities would have been his positions on issues, and it’s issues that he is most excited about discussing. Biden has some issue-based vulnerabilities, but it’s the other questions–is he up to it? Has he been in the swamp too long?–that are nagging voters. And Biden, for his part, seems more invested in talking about Trump’s indecency than about his own platform. (My guess is that preference reflects his own self-conception and interests as much as any strategy.)
So: Less Medicare-for-All, more Burisma.