I don't even think it's that. Harry thought he'd been cut off, and probably felt like he'd been cut off, even if he hadn't. So he wasn't counting on that.
But he doesn't strike me as the sort of person who'd sit down and go, "this is the sort of life I want, the legacies I want to leave my children, how much do I need to achieve this and how do I go about earning that reliably"?
(He's very emotional about that inheritance - the money from Diana has to go to his kids, not, you know, be spent on raising them?)
Meghan, I would credit with having the hustle/grift/aspiration to keep doing "stuff" but without a plan for how "stuff = money" and "lifestyle = money".
(My own spreadsheet on the matter is meticulous, but then it has to be, because six or seven figure sums won't ever appear unless I work at it)