I'm in a similar position to you- FTTC since about 2012, original modem, Asus router. Speed at first was ~68Mbps, gradually dropped aver the next few years to low 60's as the cabinet filled up. Then everyone was moved to a bigger cabinet and (presumably) the wiring tidied up- back to around 69Mbps.
I'd agree with MHC, probably down to other external factors rather than a tired modem.
Possibly of interest/relevance- we recently had a series of power cuts (intermittent and tricky-to-find cable fault). Each time the power was restored I could get ~74Mbps, until a resync for any reason would put it back to the usual ~69Mbps.
I can only guess that, after the cuts, it was resyncing before any traffic built up to cause crosstalk, but it indicates that even an old modem can sync (and hold) a higher speed if given the chance