The tech tree as a whole needs to be redone and expanded in general, mostly because it feels a bit like getting modern-day techs in the 1800s is the norm rather than the exception, as even on mid levels you and the AI are all going to be in the Information Era well before that time in reality without real concerted effort. Really, stuff like Railroads and even Combustion to an extent should be Industrial Era.
And yeah, flight is very overpowered, simply because you can cram so many airplanes into a city. There should definitely be a limit, and the actual capacity for air units made into an actual factor. I think that, at base, any given city should start with 3 slots for air units, with an additional one added in the Atomic and Information eras. The Airport should give another three, and the Military Base three more. A city itself gains 1 more slot for each Fort in radius, and two for each Citadel, while Forts and Citadels themselves are able to hold air units, Forts having three slots, and Citadels having five. Of course, if they're pillaged or stolen from your territory, they obviously will lose that capability, but it makes sense, especially considering the late-era update that depicts those two improvements as more advanced military base installations. And of course I guess additional policies and stuff could give more slots: maybe something in Autocracy adds some, and I definitely think a revamped Pentagon that gives free B-52 Bombers (since I am in support of America having those replaced with a UB/I) and an additional two air unit slots in each city would be a good idea. If Future Tech is ideally made to do things, it could also add more. As it stands, it seems a bit heavily unbalanced that any crappy town can hold hundreds of combat airplanes when those have always been extremely complicated machines requiring specialized facilities to maintain and deploy them.