Of course Charles will be king but the he is the most 'experienced' heir in history argument to me is overrated. How does this experience translate in a parliamentary monarchy? Charles has no executive authority. Royal prerogative is exercised through government Ministers.
If William became King tomorrow he would have more experience than his grandmother when she came to the throne.
That's not entirely what I'm trying to say.
For starters, in Charles we have a man who has been engaging with the public in various forms for 60+ years. He's had some highs, he's had some serious lows. He's done well in some ways and made some serious mistakes - but through it all he's weathered the storm. He's learned not to expect eternal love from the people, but he's also learned how to come back from being hated by them. This is something the Queen has also learned (although not to such a great extent), but is not yet something William has had to learn (and one would hope that he won't have to).
Secondly, in Charles we have a man who definitely comes across as one who has learned how to take great pride and pleasure out of royal engagements. I definitely don't think that he enjoys every engagement he does, but he's found a way to work what he's passionate about in life into his duties (through the Prince's Trust), while also doing the more "tedious" ones. He's shown himself to be a hard worker, and I think we can expect that to continue when he is King. Camilla certainly hasn't achieved as much in that regards, and likely never will, but I think she's also developed a good number of charities that she is passionate about and uses that to push her when she's doing the more "hum drum" type of engagements. I do think William is starting to find this - with Centrepoint, Tusk Trust, and Heads Together - but I also think that he puts a lot of his 'passions' towards things that aren't connected to his royal life - his children, his work with the EAAA, and previously his work with SAR. I don't think he should be denied this, but I do think that he would benefit from having more time not as the monarch to give himself more time to grow into his royal role. I also think that Kate will benefit from this (especially as the children age) and she has time to ease into her royal role.
Thirdly, Charles is going to have some authority, in a manner. The Queen does her red boxes almost every day of the year and meets with her Prime Minister weekly. If you think that doesn't hold any power you're naive. She might not be able to go "off with his head" and have that followed through, but she does have a voice that is listened to, which is in and of itself extremely powerful. Charles has experience with the red boxes and has been receiving his own version for a number of years now. He knows what's in them and what's expected of him when it comes to them. He might not be sitting in on the weekly meetings, but I have no doubt that he's been taught about what to expect from them, and I honestly think that the black spider memos are really just him as PoW engaging with members of the Government in a way not too dissimilar from how the Queen engages with her PM. I believe William is beginning to get his own red box equivalent, or at least training on them, but again I don't think it's comparable to the experience Charles already has.
Lastly, yes the Queen was younger than William is now when she became Queen. And I think it shows. If you look at the early years of her reign, while I think you can see some vitality to her reign that the end of her father's reign lacked, you can also see mistakes that she made largely because of her age. There are steps that she was forced into taking because she was a young, inexperienced woman up against a group of older, more tested men. There are times when you can clearly see her trying to force her own hand in issues, there are steps she made that you can see she was very likely overwhelmed with her role. Issues like the name of her House (and her children's surnames), the relationship of Princess Margaret and Peter Townsend, her role in the Suez crisis, the way her children were raised (particularly Charles and Anne), and the balance between being a mother and a monarch, even the alleged marital problems between the Queen and DoE during her early reign.... all of these very likely would have been handled differently had the Queen been older and more ready for her role when she became Queen. Now, of course we're not going to see some of these challenges during Charles' reign - due to his age, due to the way the world has changed in the last 60 years, due to his gender - but some of them are ones that William might be exposed to, which is why it would actually be beneficial to the monarchy to allow, if possible, William to grow as a person more, to allow his marriage to mature more, to allow his children to spend more of their lives not as the children of the King.
Older, more mature monarchs who have older children are better equipped, in my opinion, to provide stability to the monarchy than younger monarchs with younger children.