I remember reading an article (can’t find it right now) from a PHD dropout who was doing research in string theory. One of reasons he dropped out is his frustration at how abstract and disconnected from reality his work was. His advisor (and his colleagues) didn’t have that problem, because to him, the math behind string theory was an ends in itself. There’s beauty in math, regardless of whether it has any practical application. If string theory turns out to be an accurate model of reality, then that would be a nice bonus, but that’s not why his advisor studied it.
So to answer your question, if we somehow reach the point where everything that can be feasibly discovered has been discovered, then theoretical scientists would make up their own models and study those.