I think its pretty clear IMO that Anne's children won't take up royal duties and are making their own way in the world, similarly the general opinion seems to be against Beatrice & Eugenie doing full time royal work. I think its fair to guess that as the chose not to give them titles, Edward and Sophie are planning for their children to go out and work in the real world (or as real as it can get for the grandchild of a sovereign). That will in itself slim down the monarchy in the future.
By the time William is king (imagining Charles to have a relatively short reign), the Kents and Gloucesters will probably be happy to not do much more apart from things they are really very much personally interested in.
Even if Anne, Andrew and Ed & Sophie want to carry on with duties they will be that bit older, especially Anne, so I can imagine them only doing things they are personally interested in, perhaps cutting back on some patronages and not taking on new ones. I can imagine Andrew being stopped from doing much overseas work and maybe Ed & Sophie's work for the DofE awards being paid for by the charity in some way so its one less thing coming out of the RF finances (I notice this year they are off to the Bahamas or somewhere exotic for the DofE Awards so I can't imagine William and Kate's Court being that willing to pay for trips like that)
In effect by William's reign we will have:
- King William
Princess Consort Camilla (possibly and what a ridiculous title btw)
The Prince of Wales, Prince George
We could possibly add harry's children but they, and possibly even George and Charlotte, may follow William & kate's well rested footsteps and not take on full time royal duties until they are middle aged by which point Charles' siblings will probably be very much near 'retirement'.