The alternative candidate(s) would be attractive, well-spoken, and well-rehearsed to claim surprise and although reluctant to serve, will do so to preserve our "democracy". Triangulation might be likely (meaning that the candidates could say that "Joe went too far" in places.)
Am I certain of this? Of course not, it's merely a guess. That said, we should prepare for the contingency by making strong, popular policy platforms now, and expecting to run against a pretend middle ground, and not just JB.
People say that, but I do t see Kamala going quietly, or remaining second banana quietly