The figure of 1000 to 1500 royal duties a year is just ridiculous. Especially since at their very peak, Prince Philip and Princess Anne carried out around 670 royal engagements a year and both have reduced, Philip due to age (he's down to the 300s) and Anne probably seeing that it wasn't sustainable, she's down to the 500s. Even the Emperor of Japan who is the Head of State of a country of 127million people at his peak carried out 1000 engagements a year, in his case his workload has been reduced as well. Catherine as wife of the heir of a country of around 60 million won't be expected nor able to carry out engagements into the 1000s!
Catherine will be 40 in 11 years time, (she turns 29 in 2 weeks) so by the time she is 40, Andrew will be 60 and carrying out his full load (500) engagements a year, Edward and Sophie will be in their mid 50s so their load of around 300 and 200 engagements will carry on. Anne will be 70 and probably be down to about 400 engagements. The Duke and Duchess of Gloucester in their mid 70s and so carry out the 200 and 100 engagements they do now. Assuming good health the Kent siblings will be in their 80s and probably still doing a handful. This not not even counting Harry (perhaps wife) Beatrice and Eugenie, Charles and Camilla.
Fewer royals, just means fewer organisations will have royal patrons, not that the remaining ones drastically increase the amount of engagements they have. An example of a royal patronage going, Diana was patron of Barnados, she gave it up, and Cheri Blair as wife of the PM became patron. Now Barnados has a royal patron again with Camilla. The older royals such as Prince Philip and the Queen also have numerous patronages that are not UK based, Charles has a few, the younger royals' patronages are all UK based. I don't think it's as likely as it was in the past for organisations outside the UK to ask a British royal to be their patron.