A widely-noted feature of the debate over the monarchy, both in Australia and the UK, is that the Queen is considerably more popular than the Monarchy. Although there are a lot of factors of work here, one is a curious accident.
In a system where the monarchy is inherited by male primogeniture, the par outcome is that the monarch should be an old or middle-aged man who has spent most of his adult life waiting for the job. This was the case for most of the 18th century, for example. By some odd chances, however, more than half of the last 200 years of the British monarchy havs been occupied by two long reigns by women. As at the end of the 19th century, only a minority of people can now recall a time when the current Queen was not reigning. The only reign of the last 150 years that really fits the model I’ve described was that of Edward VII, and was not a great success.
In the past, when most positions of authority were held by men whose main qualification was to have lived a long time, the operations of primogeniture helped the monarchy, although even then the really successful kings were mostly those who started young. But I doubt that a modern monarchy could survive if the par outcome occurred regularly. Assuming the British monarchy sticks to the existing rules and Prince Charles doesn’t die prematurely, this is the likely pattern for the foreseeable future.