Bernie got 43% last time, and double the #, and was winning people that were basically people willing to give the party one last try.
Now, he is winning around 30% with a coalition that is far more loyal to the party and that has shown less likely to vote for Trump or stay home.
You can keep thinking that...
The percentage point difference (which won't be apples-to-apples this time, for obvious reasons given he isn't staying in the race through June + many contests that would have already voted moved to June) between 2016 & 2020 (using the 30-43% gap here, which is dubious, but whatever) was like half attributable to 2016 protest-voters who weren't going to vote Democratic regardless; a good 6-7% of '16 D primary voters were Republicans locked into the process and voted against Clinton. On that part, you are right in calling them "Republicans". Between '16 and '20, what will make up the remaining difference between 30% and 43% won't be Obama-Trump voters, but people who just switched to Biden because Bernie was effectively done. There is a meaningful number of people who just want to be on the winning team in elections.
If anything, the fact that the Sanders coalition this time was far less comprised of olds and whites means the core of it will be
less reliable to the Democratic Party, not more. Arguably the single-biggest difference in the coalition between '16 and '20 was that Sanders supporters this year are disproportionately comprised of the future of the party: Latinos and Asians. These groups are disproportionately young and low-propensity, which makes them more likely to stay home or even protest vote in the GE (i.e. stop attributing black voting behavior to all non-white groups). Once you jettison the GE-Republicans from '16 locked into Democratic primary because of ancestral and closed-state dynamics, there's no way to argue this year's coalition is inherently more loyal.
People are also ignoring the abstract dynamic that many Sanders supporters have now lost not just once, but twice - in conjunction that many considered Clinton heir-apparent and Sanders' run in 2016 a suicide gambit, whereas this time, he was seen at various points as a clear frontrunner. This will create a greater sense of bitterness among a meaningful number than the loss in '16 did.
