I think Kate, William and Harry's roles are all also limited by the need to future proof their schedules.
We know that Charles' intention is to have a streamlined monarchy and likely to change to some degree the type of engagements that are done. But trips to various parts of the country will still need to happen; the reduction is going to come in patronages, I suspect, not "visiting Colchester to view and be seen". If Charles was going to cut those out, he wouldn't still be doing so many of them.
So say William, Harry and Kate did increase up to 350-400 engagements a year, today (leaving aside the question of where the money for that is going to come from). And then HM lives for 10 more years. They'll have "locked in" 350 engagements and won't necessarily have the time to pick up more once Elizabeth, Philip, the Kents, and the Gloucesters stop doing them.
The queen and Philip aren't going to stop, nor are they going to ask HM's cousins to stop. And, frankly, that's to the good. I know a lot is said about how the older generation should be allowed to put their feet up, but a) they clearly don't want to and b) it's likely why they're all comparatively healthy and active. (I say this as someone who's watched her grandmother volunteer for close to 20 years now; Grandma's 99, still actively volunteers 2-3 days a week, and looks about 10 years younger than she is. I'd never dream of telling her to slow down.)