I'm not saying that there's no need for maneuverability, just that sustained 9g is probably beyond the point of diminishing returns. It puts great strain on the airframe, which requires extra weight to be strengthend for it. it also wears out the aircraft sooner. One of the reason the USN started restricting the g that should be pulled on its fighters was to extend the life of the airframes. 9g also puts a bigger load on the crew, with the possible of momentary or (relatively) extended G-LOC. Both F-20s were lost because of this, and no doubt other a/c as well. 7-7.5g is probably enough.
We are coming through an age of technological surprise in air combat as great or greater than what the Argentinians experienced in the Falklands War with the introduction of the AIM-9L. Up to that point, air combat techniques dealt with primarily getting on an opponent's tail because (except for long range missile shots), you really needed to be there to get a good shot, be it missiles or guns. Defensive maneuver tactics focused on preventing your adversary from getting into that position. What AIM-9L did was allow someone to take a Sidewinder shot from any aspect, and the tactics of the time were not up to dealing with that.
Similarly, we have now seen the introduction of high-agility, high off-boresight missiles, often cued by a Helmet Mounted Sight (unless you're the F-22). These have large "no-escape" zones, wherein there is no maneuver the target aircraft can make that will defeat the missile. A defensive break turn is still something you need to do, but the effectiveness of sustaining it at 9g is probably not all that much better than what you will achieve with 7g against a weapon coming at you from any angle that can turn at 60g (or 25g for a long range missile), especially one that may still be under power at the time.
I'm not saying that the maneuverability of a 747 is good enough (unless he has a multi-shot trainable directed energy weapon
and that's still in the future), just that 9g probably no longer justifies its cost.
BTW, the F-8 owed its best-of-the-war win/loss ratio mostly to tactics and training. F-4 crews had to learn ACM, but also long range intercept, various kinds of ground attack and in USAF's case nuclear strike. Plus, F-4 crews weren't trained that much in the concept of maneuvering combat in the first part of the war. F-8s did limited bombing, but first and foremost they lived, breathed and trained for air combat. Arguably that's why they had the best win/loss ratio up through 1968. F-4 got to be a lot better once the crews were trained to use the F-4 strengths, including acceleration, fighting in the vertical and to force the MiGs to sustain their turn (where they ran out of energy).
