The duchies are how Charles and William fund their staff salaries, their security, travel expenses, the expenses involved in running their charitable foundations, and fund their families and eventually their families' families. I think they also use the funds to upkeep various buildings. It's a lot of money, but they have to pay for a lot from it.
The duchies are also a way of stewarding the land. If not for them, more land would be sold and built on.
It's annoying that Charles and William get so much money, but on the other hand, why would anyone put up with the life they have if not for the riches? Don't forget that the most intimate details of their lives are prone to appear in the press, they're subject to frequent harsh criticism, they have no freedom whatsoever to just go about their lives without the press analysing them and writing negative articles about them, they can't pop out for a pizza while looking in a state, they can't just go swimming without the risk of being kidnapped or attacked. They're absolute prisoners. Money cannot make up for these things. Add to that, they cannot realise any personal ambitions at all. Their whole lives are devoted to serving the state. Fancy being a lawyer or a nurse or a detective? Tough luck! Any talents you might have will go to waste.
And I can't imagine how awful it must be to have your life dissected on the front pages of the paper. All the personal details of your life laid out for the whole world to see. Your parents' divorce, your bereavement, your problems with your brother, everyone in the world reading all about it. Their whole lives are a recipe for mental-health problems. Money doesn't help with any of this.
I think we're lucky they continue to serve our country, instead of telling us where to shove it. Without them, we'd be in danger of being like America, with Trump.
Just imagine the UK as a republic with a President Farage. Charles and William not looking so bad now, eh??
I really like having a monarch who's above any greasy politician.