What if 20% of Biden supporters wouldn't support Sanders?

[Don1]

To be clear, in any election there is a small number of registered Democrats who don't go from one candidate to the other after the primary and that is NOT what is being discussed here. Instead, the VAST MAJORITY by far are Democratic Party-leaning Independents, and possibly even some Republican leaning independents.

If that many Biden supporters were in favor of Biden but would not vote for Sanders in a general election, then the Biden campaign would be congratulated on having a broad base. It would be said that independents and moderates have a preference for Biden relative to other candidates. It'd be considered a success.

In fact, it's already considered a success in a lot of ways that Joe Biden has appeal to quite a number of independents.

I agree that this is a measure of success and I will go further that a good candidate will have a broad coalition among independents, moderates, progressives, and a strong energized base ideally.

(This is not to say that registered Democrats only willing to support 1 candidate is a good thing because all Democrats should vote blue, but this is not about registered Democrats.)