What needs to be modernized?
https://www.oig.dot.gov/sites/default/files/WEB_ATC_Mod_4-14...
Basically, FAA's nextgen project has been some big dig level crap (https://en.wikipedia.org/wiki/Big_Dig for those not familiar)
They've been trying to figure this out for almost 3 decades (though publicly, the current incarnation only started to have a joint planning office in 2003, and so they try to pretend that's when they started).
They currently don't expect to finish until 2030. AFAIK it'll have been ~50 years for a technology modernization project.
(I actually don't know all the details, so if someone wants to come along and say "it's not that bad", great!)
Good watch if you wanna see how naively designed the whole thing is.