In this case I suspect you may be wrong.
Like a speaker, most cartridges do have flexible parts, specifically the suspension that holds the cantilever in place but allows it to move.
These parts are made out of rubber, or some similar flexible material, and probably will in fact change after the first few hours of use...
(They can also eventually weaken due to fatigue or harden due to age and environmental exposure.)
And, when the characteristics of the suspension change, the resonance and compliance of the cartridge, which are partly determined by the suspension, will also change.
Therefore, it's not at all unreasonable to expect a cartridge to both "break in" and "wear out"..... to some degree.
(I would expect some cartridges to be more subject to this effect than others - depending on the exact materials they use.)
With speakers, and speaker drivers, measurements and specifications are assumed to apply "after a reasonable break in period".
Note that, with speaker drivers, playing music somewhat loudly for 12 or 24 hours is usually considered to be sufficient for this.
(Also note that very few electronic components experience any sort of break-in... and most circuitry is relatively immune to change when and if they do.)
It's worth noting, however, that we humans are also subject to "break-in"
(There is a technical name for this effect - which applies to all sorts of things - including our perceptions of color, loudness, and temperature.)
When we first hear something new, that has a specific sonic signature, different from what we currently have, it tends to sound "odd or different".
Then, as we listen to it for a while, we eventually stop noticing its specific sonic signature (unless it is specifically objectionable).
Once that happens, and we no longer notice it, we have fully adopted the new device as "our new normal".
At that point, our new device "just sounds normal", and anything that sounds different sounds "odd or unusual".
This effect can be very powerful... and undoubtedly accounts for many claims of "equipment breaking in".
Ha Ha!
You CAN break a record but you can’t break IN a cartridge. Actually you need to be older to remember broken records.
Don’t believe me about the cartridge break in falsehood..? ..Just get one of those exotica cartridge companies live on the phone. They’ll tell you anything you want to hear.
Bill