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

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.