I think you have a point here - a lot of people already want William to replace Charles. My mom asked me this the other day; she thought the Queen should 'skip Charles for William' and doesn't understand why she shouldn't or can't. I explained it to her, but of course there are many people out there who don't follow the monarchy's traditions closely and only know the 'pop culture' version of royalty that they see in glossy magazines and entertainment shows.
So if the idea of Charles becoming king over William is already unpopular, I don't think it will get any better if he's an 80 year-old man. The Diana story will get dredged up again, the suitability of Camilla to become Queen will be debated in popular media, and this will all contrast with William and Kate and their stable family and relative young age.
I don't think there will be an abdication really, but I do wonder about a regency. Although I wonder if it's too late already to draw people away from their preference for William.
You very much have a point, that applies to most monarchies (the Dutch being an exception), that is that in contrast to earlier times monarchs nowadays can expect to live long. Which means that every second heir will be "old" by the time he/she can take over. Having spend most of their lives in a kind of limbo and when they finally sit on the throne their subjects find them irrelevant because they are too old and haven't been on the throne long enough to make a lasting impact.
That is very much the problem for Charles.
It's not the problem that he is unpopular (that may also be debatable BTW) or Diana is still casting her shadow over his life. The problem is that Charles has been waiting so long that his ship has long since sailed.
Even if QEII should die tomorrow Charles will only be seen as an interim figure and he won't have the time, and probably not the energy either to really make a lasting impact on the "firm" let alone Britain. (Keep in mind that QEII has been blessed with an extraordinary good health.) Charles will be Charles the Old, rather than Charles the Reformer, or Charles the wise or even Charles the Blunderer. He will be a footnote in the minds of most people. - Because no matter what it's W&K who are the stars of the show. They are the ones people look to. They are the ones people follow. They are the ones who have the energy to be seen here there and everywhere. Simply because W&K are now in their prime.
They are the ones most people can relate to age-wise, family-wise, lifestyle-wise even, while Charles is for the OAP's.
Okay, I'm putting things a bit on the edge to illustrate my point, but I actually mean it. I'd say to most people >50 Charles is far less relevant than W&K.
So yes, it does make good sense to "skip" Charles.
That QEII should abdicate for other reasons than health, so that Charles should get on throne, is IMO too late. Such an abdication should have happened 20 years ago.
It's too late now. It makes no sense to abdicate in order to have someone younger and with more energy than yourself on the throne if that person has a himself reached the retirement age.
It makes much more sense to abdicate and have Charles renounce the throne in order for W&K to take over. - Then they can have the time to make a lasting impact and they can have the time for people to develop a similar devotion to W&K as people have to QEII.
So should QEII die tomorrow Charles would his country and the monarchy a great service if he only reigned for max ten years before abdicating himself. That would give W&K time to look after their children in their early years, before taking over in earnest. - And Charles would still be able to advise his son.
And it's a problem in other monarchies. We have CP's who are now approaching 50. (Denmark, Norway and to some extent Sweden). The CP's there are in their prime. They are increasingly, if not already, the ones most people look to. Yet the monarchs in those three countries are in fine health and can be expected to be around for many more years, if for no other reason than that they have access to the best possible health care.
Or should they do like the BRF? Wait and wait until the grandchildren are ready to take over?
And how about the future? The average lifespan is increasing. People born today can fully expect to live until 100, perhaps longer, and still be in pretty fine shape. - Are we to have monarchies where the average monarch is 90 years old? Having ascended the throne at the age of 80?
Wouldn't it be better to have a system where the CP's take over around the age of 45 and reign for some 30 years?
Abdications have taken place for generations in the Netherlands. Yet, the NLRF is still around. It has recently taken place in Spain, Belgium and soon Japan without any sign of the monarchies there come tumbling down.