Newsom isn’t known nationally. His favorable 39%, unfavorable 24% with the rest not sure or don’t know who the heck he is. This is where the primaries come in giving an unknown candidate from January through June for the public to get to know him, who he is, to recognize his name, etc. Newsom has lost those six months even if he replaced Biden. Even if Newsom was nominated at the Democratic Convention to replace Biden, that gives Newsom a scant 2 ½ months prior to the election. The Democratic national convention isn’t until 19 August. He’d be an unknown entity to 30% of all Americans, but that might be a good thing. Both Biden and Trump are very well known, everyone knows who they are, and everyone has an opinion of both of them, mostly negative for both. Perhaps most Americans would be more than willing to give an unknown a chance over a very well-known disliked Trump. Who knows?
As for history judging presidents, no one knows how that will shape out. Truman was very unpopular in 1952, he couldn’t have gotten elected dog catcher. He knew that and withdrew from the presidential race. He had only a 32% job approval. He was toast if he ran. But history now rank Truman as a near great president, the 6th best behind Washington, Lincoln and FDR the three great president and then Teddy Roosevelt and Jefferson as the other near great presidents. Eisenhower was another president not given much credit at the end of his presidency, ranked 21st out of 31 presidents at the end of his presidency, he’s ranked 8th today.
Biden has a job approval today of 40%, most Americans don’t like the job he’s done. But how historians will view his presidency remains to be seen.