They've painted themselves into a corner. British tabloids and social media would delight in them downsizing to save money so I suspect it would feel like a humiliation. Harry should have enough money to pay off his mortgage but they're not going to be able to afford security for, what, all four of them for 40 - 80 years. He may feel the risk to A & L will reduce once they are established adults which would reduce that range but I think he'll always feel the need for his own security. I would if I were him. Because of his very high public profile and his army service ( and, now, his blabber mouth).
Overall I'm critical of many of the decisions H & M have made but I do feel a bit sorry for him on this. If I were Charles, I'd give him the equivalent cost of the additional security they would have needed had they lived quietly at Frogmore, if they hadn't courted controversy or increased the terrorist risk angle (talking about his kill numbers, etc).
By additional cost, I mean, if they were living quietly on the secure Windsor estate, that by itself wouldn't incur a cost. But they'd need security for when they left the estate (I'm thinking of them staying locally not gallivanting round the world). I think that cost should be met by Charles from his private funds. Or, again, that's what I'd do if I were Charles.