The biggest problem with sequels is exactly this - there is usually no big rewrite of the code so basically the only things that change are the bells and whistles. Very very rarely do sequels go back to the drawing board and make the effort to actually improve things from the ground up. PG2 is being rewritten from the ground up and has all the benefits that go along with that ie faster, more stable, better visuals, completely new UI etc. It just takes time with a small team.
BTW Key areas of RFactor 2 were completely rewritten (tyre model for example) and so it falls between the two extremes.
I'll take the game left 4 dead 2. lots of rewrite from the first. ten year old game that is very popular on Steam today. what this golf game could have in common to make it more successful would be a workshop. I know we have the courses, but imagine a place where one could pick up clothing and many other items designed by a community run by Steam.