Before posting a negative answer/remark, please ask yourself "how many minutes in your flying career, you have hand flown a commercial jet at high altitudes?". Try maneuvering the airplane in simulator at high altitudes and judge your comfort level yourself. See the airplane reaction.
Commercial airliner? A few hours. RVSM has made that practice illegal for the most part.
Tactical jets (A-4 and A-6), which are MUCH less stable than a commercial airliner? HUNDREDS of hours! The A-4s I flew seldom had working yaw dampers, much autopilots. Virtually ALL was hand-flown. The A-6 had a better autopilot; but still failed quite often, compared with the 747.
Still, in over 13,000 hours over 40+ years of flying, I have yet to see a "severe control issue" or ANY crash due to high-altitude flight.
We're still waiting for your examples of "severe control issues" and crashes attributed to "Flying at usual cruising flight levels"...