In my line of business (aerospace) the
material fatigue - both of metals/alloys and composites - is a very real thing. And it is dealt with constantly - both through controlled (ie measured) pre-conditioning and through continuous monitoring (again, ie through measurements - observing change in certain material properties).
As such, initially I was totally receptive to this “speakers break-in“ paradigm... However. Just like many point out, would this effect be real, there would be available, published measurements of this break-in - in speaker diaphragm properties, recorded frequency response, or properly conducted (blind) comparison listening. By all the abundant audiophile industry R&D and the associated applied academic research… However in the absence of such measurements - to my personal knowledge - I _personally_ tend to believe that such speaker break-in is negligible and imaginary.