We don't know that a Newsom campaign could beat Trump and I think we would be foolish to assume so.
Generally, if the VP doesn't win the nomination, the party doesn't stand a very good chance of winning the election. I think if Biden didn't run and Harris did not get the nomination (I think Newsom would slaughter her), Trump takes 2024 fairly decisively.
I think the democratic party's best path forward is an uncontested Biden/Harris reelection, and then instead of running in 2028, Harris decides to retire.