The problem is that beyond a certain height level the drawing code of simutrans breaks. Each time there are a certain number of extra height levels renders in front an the back to catch thing like high foreground mountains and planes or deep depressions in the back. With even more height levels, these things will break, and one would need to render even more (in most cases invisible) tiles. So teh height window gives an indication when the graphics engine will start to lead to artefacts.
However, the map window could of course autoscale to always show the full height. I would not introduce config values for that, just change the height routine as you did using the stored values of the world.