The current goal is to provide functionality, that is, have shops, guests, staff, rides, coasters, litter, etc etc.
The Gui is required to have an interface to that functionality so it can be tested, but currently nobody is working on making the Gui complete (many things in the Gui are not finished, or not working).
The current style is thus random, things get added at some place since the interface is needed.
The obvious advantage of something like the original is that people will recognize it, and be able to use it without much trouble. On the other hand, I do not feel bound to that; if better solutions are found, I see no reason to ignore them.
As you can see, lots of work to do in this area! :)