I think as candidates Clinton, W. Bush, Gore, Obama, McCain had sincere support overshadowing the need to stop some particular bad person instead. As misguided as I think it is, Trump voters also are all about Trump less than stopping Biden. I can't personally remember a race where "the other side must be stopped" as pretty much the sole consideration among the voters until the Trump era.
Yes, third party candidates are dismissed in a self fulfilling prophecy, but also that reality drives most reasonable would-be third party candidates to one of the viable parties, generally leaving third party candidates that wouldn't be that popular anyway.