The weakness is in the regulations, not the aircraft. Airliners are not designed for these kinds of manoeuvres.
I believe the NTSB report says the fin failed at twice design load; the actual max may have been higher, but let's assume that the regs get changed to effectively double the design loads requirements. To be consistent, you'd have to follow a "carefree inputs" philosophy in all axes, so let's double ALL the manouvre loads.
Now, manoeuvre loads don't always design the structure, but if they did, doubling the load will generally require twice the thickness or strength. We just, in effect, doubled the structural mass of our design, at least for primary structure.
So we've grounded every existing airliner - because no amount of beefing up will double the design loads - AND ensured we can never design an economically practical one to replace them, because they'll be far too heavy.
As to your last comment; that the plane be operated according to the procedures in the manuals is the only assumption one can make. Every single limitation, caution, warning and procedure in the AFM is a reliance upon the trained people in the pointy end operating, literally, 'by the book'. if you don't then all kinds of nasty things happen.
Suppose you have an engine failure. What prevents you applying full rudder in the wrong sense, causing the aircraft to enter uncontrolled flight? Nothing except training and airmanship. if we restricted the rudder so that you couldn't apply it "the wrong way" we'd end up making the input automatic - the only way to prevent pilot control inputs from potentially catastrophic consequences is to prevent pilot control inputs - PERIOD. FBW envelope protection or full automation. As long as the man is in the loop, and in control, he has the ability to cause a tragic result. It's inevitable.