I actually don't think anybody is screaming for that, I think that's just cope. People want Harris because they want a living person to beat back against fascism. I don't think that's a crazy ask.
Then that should be the argument made. Most of the calls I see for him to step down or be replaced and skip over Harris and go into fantasy land. It is Biden or Harris end of story.
I don't know how many people are screaming for a brokered convention specifically, but plenty of people are out there throwing out names that are not Harris, and I'm not sure how any of them would become the nominee at this point otherwise.