Others may confirm that they have similar experiences, but, the explanation of the cause will just be guesses. Your guess sounds credible. Another guess would be that the suspension damping rubber donut has hardened and is now restricting the movement of the cantilever. At this age, it could be a combination of causes.
ADC moving-iron cartridge has "low" output
I have two identical ADC gold-body XLM-III carts from the late 70s. I purch'd them used, as part of two turntables that were each purchased from different parts of the US. Each cart has a different model of stylus (XLM-III and QLN-2) , and styluses can be interchanged.
The MI series from ADC should output 5.5mV, which I have not measured. However, BOTH these carts are challenged wrt output volume. Not only do I have to crank up the vol. more than 1/3 above other carts, but the sonics are a bit "small" and not "full bodied". It might be due to weakening permanent magnetism -- these are both getting close to 50 yrs old .
The perm. magnet in ADC moving iron series is mounted in the removable stylus, just above the stylus/cantilever assemble.
Not sure what else the low volume may be due to??? These are my my only MI carts in my collection. Anyone know more?