One thing that has not been brought up along with the amount of working royals is that we don't know just how royal engagements will be scheduled and performed by the time William's children are old enough to take on the roles.
For instance, when Elizabeth became Queen in 1952, it took months for the Queen and Philip to visit other countries. Now, a royal engagement by a member of the British royal family can be done in Norway with being home in time for dinner. With the onset of instant communications and fast travel, global events can be scheduled and pulled off easily. Invictus Games is a prime example of this.
The days of individual, single visits to places to unveil plaques may even be fading into the sunset as more organizations band together under the umbrella of a royal foundation.
We just can't assume that the level and ways of engagements we have today for the working royals are going to be the same 30 years from now. We just have to look and see just how much our world has changed in the past 30 years and the differences that have come about because of those changes.