You have to be very careful to choose your optimization metrics well. "Reusability" as a technology is fantastic because it let's us put more payload in orbit. Optimizing for "minimum engines thrown away" or "common design" is bad if it hurts the ability to operate.
Starship is on Raptor 3 and still no where close to mission ready. Since 1981, a total of 46 RS-25s were used for the 135 Shuttle launches and what is coming up on the 2nd Artemis flight. Starship threw away more than that in only 2 flights.