There seems to be confusion on what "privatization" means as it is currently being proposed. As a not-for-profit, with user fees (mainly paid by airlines) ATC will have an actual steady stream of income to plan for system upgrades. They currently operate under the whim of Congress and their funding which isn't consistent from year to year.

The implementation of next-gen has taken much to longer than it should have under the current system. Aircraft are required to have ADS-B but not all ATC facilities have the equipment for it. That allows for tighter separation which provides greater capacity.

Implementation of Data Comm is another example. Slow to be implemented that allows greater capacity.