Build a lego tower. Add some pieces to the top of it. Now Rebuild the tower with less pieces without taking the tower apart. It's not a perfect example but I think it gets the point across.
That's unfortunately not a good analogy for coding. Unless they are completely inexperienced idiots (they aren't) they would have designed the UI system using basic data structures that they could modify the number of elements within it. Since neither of us is privy to the details of their specific engine we can't say how hard changing the location of function calls would be. Given that SEGA would not have been totally inexperienced when they wrote their inhouse engine I find it pretty unlikely they would have designed it to be completely infeasible to modify.
Um. I think you need to look at this a different way. This is not a matter of multiple elements. This is a matter of 2d and 3d space being tied to code, that calls other code, that calls other game objects/ui elements. Go take a look at a unity menu tutorial or two and then think about what a less standardized engine from the early 2000s designed to run on handheld consoles and might be like to work with. Modern engines like unity, cry engine, and unreal handle things devs used to have to set manually in many cases.
This is a ue3 tutorial which is given it was made to be a product will be leagues more user friendly than what sega was using at the time. http://www.hourences.com/tutorials-ue3-uiscene/