It is not over 50 cases, it is thousands of cases. But they were not reported since they are a non-event.
I would agree if the aircraft has natural positive dynamic stability, then it is a non event.
If left alone after a disturbance, it will naturally seek its trimmed angle of attack.
For an aircraft with neutral static stability, then it is a problem.
Any deviation must be recognised and corrected early. Once AF447 had deviated by that much Altitude, it didn't matter how hard they pulled trying to regain FL 350 - it would't work.
Edit: Typo: changed dynamic to static.
Thanks to Clandestino for pointing it out so tactfully.