There's the assumption infused to a lot of these conversations that Trump is uniquely bad or uniquely extreme and so that "centrism" would still mean opposing him.
I also don't get what's particularly "grass roots" about support for the Democrats. During the Kamala campaign we had a string of celebrity endorsements including a cringey Avengers reunion zoom call. These are rich, privileged people from a specific social milieu - not grass roots by any means.
The last "grass roots" candidate was probably Bernie Sanders (someone Joe Rogan also supported, incidentally), but he was too dangerous to corporate profit margins for the DNC to let him win.
