There are many things in audio that can be heard but not explained. The human ear/mind is not the same as an oscilloscope or any other measuring device. Measurements are good to have but they cannot fully inform what we experience when listening to music (and not even partially inform the emotional part of our experience).Yes, it can be measured but it's not trivial and something as subtle as cable break-in, you need really sensitive equipment and not to mention the knowledge of how to use those equipment. But the principle of cable burn in is well established (look up electro-migration) so there is no point to reinvent the wheel.
As for speaker driver break-in, it has been measured. That is why speaker designers only measure the various speaker driver parameters only when the drivers already well broken-in.