I've often thought that about Charles. He'll reign for a decade or so and then break the abdication taboo for the British royal family.
It seems a lot of the other monarchies are trending towards abdication nowadays - Spain, Belgium, Japan in recent years while the Dutch have been doing it for a few generations now. There will probably be more abdications in the next few years (perhaps Norway or Denmark). I think it will be increasingly uncommon to see monarchs die while still on the throne. Liz won't abdicate of course and I think she's got a few more years in her yet.