This is exactly what is proposed in Atlanta.
Will it happen? Who knows.
In Boston, the end of the E-line has shared travel lanes with cars running on the tracks between Brigham's Circle and Heath Street on Huntington and South Huntington Streets. I'll leave discussions about San Francisco to someone more familiar.
Outside of the US, sharing the roadway does happen – the Kustram in Belgium runs along the entire coast and parts of road, particularly around Mariakirche has autos and LRVs in the same lane. Brussels, Gent, and Amsterdam also have portions of their streetcars in shared lanes, in particular Line 92 between Louisa and Fort Jaco in Brussels is almost entirely in a shared lane. (Click on the area in this link
that says "Saint Job" and you'll find the Fort Jaco end of Line 92).
Vienna, Prague, Cluj-Napoca (in Romania), Zurich, Munich, Cologne, Heidelberg, Amsterdam, and Dusseldorf all have portions of shared running between rail vehicles and cars that I have personally observed. Granted, most of them area trying to remove cars from those lanes, but that is to improve schedule reliability and running times.
Atlanta - within two and a half hours of . . . the edge of its suburbs