So if I was flying this aircraft manually, and it had a zero stick force gradient, then there would be no trim required as the speed was reduced from 250 to 240. But it would surely be easily controllable.
Don't feel too bad about this stuff confusing you a tad .. as it did me as an undergraduate for some time. (It's OK for bright lads like Genghis who pick up this stuff more easily than do we mere mortals).
Consider, with a zero stick force gradient, that the airspeed, once varying, will continue to vary hither and thither depending on the aircraft's characteristics. All very well to think about slowing down .. but how are you intending to exercise any sort of control over what the aircraft is doing .. ie other than just being along on a roller coaster ride ?
The workload on the pilot TO MAINTAIN a speed (which is pivotally important to flying) increases significantly and, in the limit, becomes impossible for the human pilot. The aircraft is, indeed, quite easily controllable (which means, in flying, that you can manoeuvre with relative ease) .. however, have a think about how you might intend to fly with any chance of doing anything else other than trying to get the aircraft to do something along the lines of whatever you might intend it to do. That is to say, stability and control go together as antagonistic bedfellows.
The TP story involved a VERY experienced and competent TP .. one with whom I have flown in the past during a training course and could only hope to emulate in some small manner. If HE finds the thought daunting then you and I would find the actual experience more along the line of frightening I suspect.
F16 FBW not considered here, of course
Therein lies the problem .. the MIL FJ driver has an in-aircraft alternate available if it gets sticky. The question is how far do you push it in an airliner, and how much testing validation is appropriate, before you get to a new limit for the electronics which you deem to be OK from a risk viewpoint ?
In reality how many lots of 4% of trip fuel saved
You pick up the bulk of the dollars within the approved CG limits and that is a sensible operating technique.
With some fancy electronic footwork and deft certification negotiation you might get a little bit further than that. No problem on a risk basis .. but, if the software proves to be less capable than at first thought .. it might provide some unpleasant surprises.