(Generally speaking. Maybe some of this is less of a factor or more depending on the system... point is you can’t say up front that old hardware is a good idea)
There's a lot of pressure to include hand-wavy references to "heritage hardware" in the risk assessments for various reasons, but it's not well-substantiated where it exists.
Combine this with incumbents looking to keep new entrants out of the market and heritage, and reuse become the operative words
The F1 and SSME are apples and oranges. They have separate uses and achievements for their times. I haven't heard of either being widely considered objectively superior to the other.
Shuttle size was dictated by intended cargo and maneuvers, so it's not really possible to say how changing size would affect it since you'd have completely different design constraints and objectives.
[1] https://ntrs.nasa.gov/citations/20150002964
[2] https://www.nasa.gov/pdf/293261main_62868main_1_pmchallenge_...
From another article on that website: "The RS-25 is still considered to be about the best engine ever made with a fairly high thrust to weight ratio and unmatched efficiency." [1]
The Constellation Program [1] that preceded SLS had two vehicles - Ares I for ISS crew and supplies after Shuttle was retired, and Ares V for occupying the moon. The missions gave some justification for needing vehicles. Their design was primarily a means to preserve existing contracts and jobs. Additional wheel greasing made it happen.
That stuff about re-using parts to reduce risk is just what people say because it sounds good, but isn't inherently true. Kind of like how "drug delivery" and "machine learning application" are go-to pseudo-justifications for many research proposals.
Constellation was eventually canceled when ISS resupply went to the Commercial Crew Program [2]. Ares V was stripped down and reimagined as the mission-less SLS. SLS is not really intended to do much more than exist for the time being.
There are a lot of factors that will go into canceling SLS - infighting between NASA centers, preserving jobs and contracts with Old Space companies, even more wheel greasing. I'm interested to see how it plays out.
[1] https://en.wikipedia.org/wiki/Constellation_program
[2] https://en.wikipedia.org/wiki/Commercial_Crew_Program