ICAO has a plan for this kind of situation and there is a good explanation of what is called "seamless" airspace here <https://www.canso.org/sites/default/files/CANSO%20Guide%20to%20Seamless%20Airspace.pdf>.
The Singapore area is not the only problem, Europe is probably the worst, but on the other hand is the area trying to do something about what are called "functional airspace blocks"