> So most of the time the compiler sees const, it has to assume that someone, somewhere could cast it away, which means the compiler can’t use it for optimisation. This is true in practice because enough real-world C code has “I know what I’m doing” casting away of const.

Some years ago i'd agree, but experience has shown that C/C++ compiler writers would rather win benchmark games than keep working code working. So it'd be nice if there was a better reason than "well, a lot of code would break if they did that".