Note that the g requirement is not "sustained" in the normal US-standard sense in that height and speed may bleed off. So it's not about air-combat training but about assessing the student's response to elevated g (and rapid onset, since the maneuver starts in straight and level flight).
Two comments:
1. Assessing student reaction to high G, including rapid onset high G, can be done in a centrifuge much more cheaply and much much more safely. USAF routinely does so to 9 G (not just 6.5 G) today, and not just for its students, but during recurrent pilot training.
2. Assessing student reaction to high G does not require maneuver entry at 15,000 ft, nor 80% fuel state. Reportedly, it was the 15,000 ft requirement that killed the Hawk. Nor does it require exiting the maneuver at 12,000 ft, nor losing only a few knots airspeed. You can pull lots of G for quite some time in basically every existing jet trainer if you're willing to lose energy (i.e. lose altitude and/or airspeed) doing so. That was pointed out by the Hawker Hunter pilots several posts back. You only need to preserve energy if you want to be able to fight after pulling lots of G.
How many sorties would be flown with this requirement during initial training? Or is this more aimed at continuation training for F-22/ F-35 pilots would you think?
"Initial" training sorties? Not too many I would think. Such high load maneuvers will probably be limited to student pilots who have progressed to the point that they have been selected for high performance jets like F-22, F-15, (and maybe to a lesser extent F-35) to prepare them for those jets and the type of flying they will do in those jets.
This is a complicated question as it relates to both the training program/schedule and to the service life of the jet. Designing and building an aircraft for such high sustained loads for a significant per centage of its service life will result in a pretty heavy jet, so I'd guess that the requirement is for not too many of such maneuvers over its service life. On the other hand, all composite aerostructures are much less prone to fatigue than traditional metal aerostructures, so that will certainly make a difference. (The Hawk has a metal wing and it was a contender until recently. I don't know what the M346 wing is made of.) On the other other hand USAF may not yet have provided a load spectrum for the aircraft over its full service life. It might be that if the load spectrum includes many such maneuvers over its service life, existing aircraft with metal wings will either require weight increasing beef up, or redesign using composites. I don't know.
While there is lots of talk of moving some recurrent training in combat jets to a trainer, there are no solid plans for that as yet. But some of the requirements of the trainer were included with that in mind. I'd guess that once the aircraft is built and serving and its full capabilities are understood (including its systems capabilities), solid plans to offload recurrent training from fighters onto a trainer will be developed. One step at a time if you will. But some of the attributes for such a trainer (like its kinematic performance and open architecture systems) must be designed in at the outset. Hence such requirements so early in the program.