I believe it is a matter of degree. For sure new speakers and certain crossover capacitators benefit from long term burn-in. Speaker cables vary quite a bit since you have enormous power, current and phase considerations so burn in there is rational.
I find that balanced line level (XLR) cables are the least variable interconnects while unbalanced low level cables (phono cables) display more differences. With that in mind then it makes some sense that break-in of sensitive components: Tubes, Capacitators, Speakers and, to a lesser extent, solid state devices, switches and even some kinds of wire is rational. The differences are, however, variable.
I find that balanced line level (XLR) cables are the least variable interconnects while unbalanced low level cables (phono cables) display more differences. With that in mind then it makes some sense that break-in of sensitive components: Tubes, Capacitators, Speakers and, to a lesser extent, solid state devices, switches and even some kinds of wire is rational. The differences are, however, variable.