Considering the dynamics of 2020 election cycle are markedly different than 2016, I don’t read as much into the “enthusiasm gap” for Biden compared to HRC. There’s a fair amount of “out of sight, out of mind” with the pandemic and societal upheaval keeping Biden less in public view compared to HRC at the same juncture in the 2016 cycle, whereas Putin’s Bitch is constantly in traditional and social media (for better and worse).
If Biden really needed to shore up support with young, black voters, they broke more towards Bernie than Kamala during the Dem presidential debates (based on polling). And the ones who skew towards Trump nowadays would likely prefer to vote for Kanye West instead in the few states that will have West on the ballot.
I like the way FiveThirtyEight grades the various polling entities and aggregates their output. Their “emergency” podcast today re: the Harris pick, and their video podcast yesterday, are decently enlightening IMO.
You ask me for "polling or evidence" and when I give it to you you say, essentially, polling or evidence doesn't matter this year. I see how this works. It took me a moment, but I think I have it now.