According to what's been conveyed to the royal reporters, Harry is not ready to take on full-time royal work, so I'm not sure being part-time is all due to William. Plus we're not taking into account Camilla's role. She only does about 250-300 engagements a year, a far cry from the other senior members. If Harry, William, Kate and Beatrice upped their workload, some would think they were overshadowing Camilla.
Good points soapstar.
I agree that the decision to be part-time royals was not one made solely by William and Harry. There are other members and factors to be taken into consideration and ultimately the preservation of the monarchy and its transition to the next reign must be considered as a top priority IMHO. The Cambridges and Harry are merely a part of the entire package.
Also I believe that the realities that would come with a slimmed down monarchy are already taking place now with QEII's grandchildren rather than with the generation that precedes them. QEII's children and cousins continue the status quo with their engagements, patronages, and ceremonial work. In the meantime, the part-time younger generation are mostly focusing their attentions to a select group of organizations that fall under their foundation's umbrella in addition to tours and their ceremonial duties: honorary regiments, investitures (William)etc..
IMHO there are two courts that are functioning side by side now. The major one is headed QEII/DoE and includes her heir, his spouse, her other children and their spouses and the cousins. The minor court is headed by Charles and is moving his children and daughter-in-law into their places for when his reign begins. Everyone still answers to the current monarch, but one group is being prepped for the next reign right now.