Let me add perhaps another way to look at this. We say, through testing, that speaker break-in does not occur. Speaker performance does not change after perhaps the first 30 seconds of use.
But we also admit the possibility that our ears must become accustomed to different speakers. I believe (if I may be forgiven that personal excess) in the process of "educating the palate". Nuances of perception that were initially unnoticed become detectable, and eventually essential, as my senses learn to perceive new input.
Is this not "break in"? Am I not better able to judge the thing that I am sensing after my brain has learned better how to interpret the signals it is judging? Does the fact that it's perceptual rather than physical make it less real?
This isn't something that could be tested with a double blind A/B test because those tests don't provide the time needed for my nervous system to learn. But that still does not make it unwise to give a new piece of equipment time to "break in" before I judge it.
Now, if that break in period just happens to take as long as the return policy, that's just a wild coincidence.