I doubt that George and Charlotte will do 500 engagements a year when their parents aged 36 don't even do half that amount
...they do - together. xx
Some things are clearly going to change in the future. Whether for better or worse remains to be seen. Although, I do find some 'accepted' notions to be rather humours or rather optimistic.
Once William ascends, it'll be down to him, Catherine, Harry and Meghan & likely George/Charlotte/Louis starting to become part time working royals (I am unsure why people seem to believe that they would not be following their parent's example of a very soft and slow entry into royal working life.)
Maybe William will tap Bea and/or Eug for a role akin to Princess Alexandra, where they are active for a handful of patronages, but not actually anything close to full time.
I very much doubt Harry and Meghan will be keen to have their children be working royals. I would actually bet good money on the opposite. While I see them be dedicated and important assets to the BRF for the decades to come, I am quite certain they'll nurture their children to find their own paths apart from the family.