I realize my questions may have been naïve and I thank you all for your patience in replying. I also appreciate the discussion.
I have tried to summarize the replies. Please correct me if I am wrong.
The flight crew chooses the best altitude for the estimated aircraft weight while respecting the limits defined by the aircraft manufacturer in the Flight Manual and in the Flight Management System. The manufacturer's limits include a margin for moderate manoeuvre or turbulence.
Therefore the selected cruise altitude should ensure a safe flight unless a manoeuvre (advertent or inadvertent), or turbulence, results in the aircraft exceeding the margin.
If that should occur, the flight crew is confident, thanks to their training, firstly that they can identify what has caused the situation (eg over or under speed), and secondly that they can correct the situation before severe buffet occurs and certainly before an excursion outside the certified flight envelope, possibly leading to an upset.