This is the crux of the matter. What may have been OK twenty or thirty years ago is not OK now - there are just too many of us and even the gentlest touch, multiplied by a factor of x, 'will eventually damage even the hardest stone'.
Quite. But in context, we're not talking about millions of people climbing on the stones. The point, in context, was whether the current rate of one person every two years mounting a lintel stone was represented an imminent risk.