She just lived to be 96.
And was doing her job up to two days before she died, as she vowed she would.
Charles could abdicate, but I doubt that he would, given his mother's example of lifelong service, and the general feeling that 'Uncle David' abandoned his duty by abdicating.
And why would he? He seems to enjoy being King, and he and W&C would probably all rather the children were older before W becomes King and George moves nearer the throne.