Do we agree that speaker drivers are not "free" oscillators? That they do not resonate at their resonance frequencies and produce tones? Instead, they are "forced" to oscillate by the magnetic-force interaction (between the electromagnetic coil of the speaker and its constant magnet)? With the frequency of the AC current (ie the sound waveform) flowing through the coil!?
Yes, in theory the internal resonance frequency(ies) of the speaker membrane can affect - eg amplify or reduce - the driving signal... However, (a) this might happen only very close to the resonance frequency and (b) even there would be negligible at the Q levels we're looking at.
So, to me the data you've pointed at does indicate that speakers' internal stiffness thus resonance frequency can/does change a bit over time. However, as this resonance frequency does not define/affect the output signal (aka the "frequency response"
) - that is driven by the efficiency/strength of the electromagnetic force - it does not have either (a) substantial audible or (b) positive ("sound improvement") effect on the music reproduction.