What is clear from reading both the accident report and that NASA history thing is that after 60+ years of development and research, the designers still really don't know everything about how a tiltrotor flies. The accident report confirms this: The computer modeling of the aircraft behavior and required pilot inputs wasn't even close in the high-speed regime. When it began to go pear-shaped, things happened FAST!
And so the challenges facing the computer guys are immense and daunting. It's not so much a matter of "Back to the drawing board!" but "What do we do if this happens again? And will simply making the fin larger do the trick for sure?" If it is true that the old fin design was just as fast and just as stable...well...that might work. But what if it doesn't?
You have to admit one thing: In airplane mode, if that thing were a normal fixed-wing you'd say to yourself, "Man, that thing has a TINY rudder!" But it's not a normal fixed-wing.
Hey, "stuff" happens in the development of any aircraft. Perhaps Agusta will go back to the (sexier!) XV-15/V-22 tail. Probably won't be as fast, but maybe won't lose control at high speed.
Either way, this accident is a sad commentary that after all these years...after all these flight hours of testing...after all these computer models...there are aspects of How A Tilt Rotor Flies that elude us. How far has this accident set the program back? I'm sure another ten...err, twenty years of development ought to do it. (I wonder if Agusta will ever tire of throwing money down that black hole?)