Biden steps down and that Newsome guy runs as a Democrat.
Biden runs for re-election against Trump and there will be a third candidate to split the vote someway.
Newsome actually wins the Republican nomination and changes the Republican party, like a lot.
DeSantis pulls a miracle and wins the Republican nomination and the election becomes a four way with Biden, Trump, DeSantis, and someone else (probably Newsome).
The RNC and DNC will only back Trump and Biden respectively. One of those two barring unfortunate circumstances will be elected because our voting systems punishes split votes and the public knows this.