I agree that William and Katherine's current workload has nothing to do with HM or the DoE's workload. In my opinion, the pair of them will continue doing as much as they possibly can until they cannot anymore (even possibly a bit after that), because that's just who they are. That's the work ethic that the pair of them encompass.
I think we're likely to see similar with Charles and Anne, as that's the work ethic that they have as well. Camilla will likely continue to do a lot, although I think that's more due to her position and who Charles is than her own desires. Andrew, Edward, and Sophie, however, are all likely to retire more at some point - both because they can, and because they're less motivated by the work.
I don't doubt that William and Catherine have permission from HM, the DoE, and Charles to maintain the level of work that they've done so far. I kind of doubt that they're being forced to do so little though. The Queen, DoE, and PoW are all such hard workers that to me it seems out of character for them to actively encourage the Cambridges to cultivate an image (note, just an image, I'm not saying that this is who they actually are) of laziness. It's contrary to their own work ethics and detrimental to the image of the monarchy.
If the Queen were to put her foot down and force them to do more, I have no doubt that they would do more, but I don't believe for an instant that this is what she's chosen for them.