There were many loud calls from establishment media and Republicans for Trump to drop out at numerous points in 2016 and they didn't work because he, unlike Biden, has a loyal electorate. And sure, if Trump was replaced by Haley she'd demolish Biden--but that's just further evidence against Biden!
I dunno, without the MAGA folks does the GOP have a chance at 270? If Trump drops out a whole bunch of them stay home.
I think one of the reasons why many of the fairly normie center right people in the GOP structure support Trump is fear of MAGA, but the other reason is they know their analytics.
I don't think most people asking Biden to drop out are doing so because he's committed some grave immorality or act of corruption like Trump. They're doing so because they want to defeat Trump.
It is surely the position of for example the NYT editorial board that both candidates should drop out. But saying Trump should drop out is (a) *pure* fantasyland, even more of a long shot than Biden dropping out, and (b) would only dilute the case for replacing Biden.