I used to take comfort in the idea that all things pass in time, now not so much. Probably because I realised that includes everyone I love, and myself!
I’ve no great love for the monarchy, but this is certainly the end of an era in British public life and likely in UK international relations - I can’t see the commonwealth nations welcoming King Charles as their new head of state.
And it is weird, there are some things you just never expect to change. I’m hardly a spring chicken, but Queen Elizabeth was not only there my entire life, but Queen far enough into the past before I was born to have interacted with historical figures (like Churchill).
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.