Modernising the monarchy has to be looked at in context of when Charles was talking about this in the late 80s, early 90s. Not to take away from any of them, but the Kents and the Gloucesters were the late Queen’s cousins. All HRHs, living in KP or wherever and getting the sovereign grant/civil list. Only didn’t happen to Charles’ cousins because it was their mother who was royal. Note how Andrew was reportedly unhappy because of those privileges being taken away from his daughters, and how Edward and Sophie purposefully didn’t style their children as HRH and there’s no suggestion Louise is exercising the option. So it has changed, otherwise the York and Edinburgh children would be working royals (or expecting to be) and we’d be directly funding them.
I know this doesn’t directly tackle the wider questions of what they do, are they worth it etc but it has changed in the past 30 or so years and Charles likely had an influence on that. And with the older generation passing and the smaller cohort of HRHs when William accedes, he may well have more options.