No matter what you think of the current gen of fighters, the fact of the matter is airframes do not last forever. Combat a/c have to be replaced eventually. It doesnt make sense to not improve the designs when you do that. Thats why despite the F-86 Sabre being a fine fighter, the USAF and NATO countries no longer use it.
The F-15, F-14, F-16, Harrier, Tornado, et al will have to be replaced eventually, no matter how good they may be currently.
I think the right question is how much better (and more expensive), do they need to be? and how does that fit with long term strategy of the services involved.
What makes a modern AF capable is not just the a/c they fly but the personnel that backs them up, the training, the combat support, the strategy and tactics involved, the weapons they employ. Despite being supposedly low tech and due for retirement the two of the most effective fighters in the 1991 Gulf War were the A-10 and the Jaguar. Neither cost 100 million dollars. Despite being subsonic the A-6 Intruder is widely regarded as the USNs best attack a/c for many decades, and so on.
Say, looking at the B-2, I think when you end up with a bomber that costs 1 billion dollars a copy, at that point it can be argued that a design is costing more than what you are getting in return, ie. value for the money.
