Modern aerodynamics and engines enable aircraft to be designed with such sustained performance that the pilot is becoming the limiting factor in how they are used. I suspect that the USAF are looking for a vehicle where they can train (and chop) pilots to handle their bodies as well as learning how to fly the aeroplane.
If you doubt the importance of this (which was not a problem for earlier generation pilots) then read the accident report on the F-22 that was lost checking its high g handling at Edwards. The new trainer will be around for 50-60 years from now after which the whole unmanned business will have become the established norm. This trainer has to bridge the gap from now until then.