[aqsalose]

Hereditary monarchy is only one kind of monarchy.

Early on, the Swedish king was elected at the Stones of Mora. The Holy Roman Emperor was nominally elected by prince-electors (who most of the time elected a Habsburg).

And even withing a hereditary framework, there are other alternatives to retirement in addition to outright abdication. An elderly monarch could for all intents and purposes retire and a let the crown prince (and I suppose in current British succession order, crown princess) rule, appointing them as a co-ruler.