Perhaps it depends on the design. An Electrostatic with high step up transformers + high voltage capacitors is more likely to change over time than a sealed box speaker with a compliant woofer. In the case of a sealed box speaker single woofer the air suspension and driver back emf will probably dominate after five minutes of break in (five minutes loud is probably enough to make the surround and spider compliant). An array speaker with 8 cones that barely move might also be a different matter - it is conceivable that some drivers may not break in as quickly as others - leading to a long break in time. Designs with weak motors and light weight cones in an under damped box where mechanical suspension dominates may change response every time you play them as the surround is known to become stiff after a day without uses and takes a few seconds to become compliant.
So I guess it "depends" - it is certainly possible to design speakers that will not need breaking in after a mere few minutes or that will not change with age. The trick is to make the mechanical compliance a very small factor compared to everything else that it becomes irrelevant. This suggests a design with big motor and beefy amplifiers that control excursion and an acoustic damped suspension will need less break-in or next to no break-in, IMHO.
So I guess it "depends" - it is certainly possible to design speakers that will not need breaking in after a mere few minutes or that will not change with age. The trick is to make the mechanical compliance a very small factor compared to everything else that it becomes irrelevant. This suggests a design with big motor and beefy amplifiers that control excursion and an acoustic damped suspension will need less break-in or next to no break-in, IMHO.