I have no idea what I'm talking about here, but how many "modern air wars" have there been in (say) the last 50 years? The Falklands war is one - how many more have there been? How many do we expect in the next (say) 50?
It seems to me that conflict is getting smaller. That is, it's less about taking over entire continents and more about killing a few people at the train station to get in the media and get some fear going. AFAIK, the wars in Iraq and Afghanistan used some air support, but there was no "air war" as such (maybe some people trying to shoot down planes, but that's entirely different from dog-fighting).
That said, if another state of reasonable size was to decide it wanted another state's land, then things scale up quite quickly. However, how much of that really causes "air wars"? The issues in Ukraine suggest that the implications of state-based action is more financial than it would be military.
When the Eurofighter project was in full swing, I wondered the same thing - I mean, how many times do we think we're going to need such a thing? That said, it came up with (what looks to me at least) as a pretty cool aircraft with some cool tech inside it. Shame it's all classified :-(