Jo's sister is a lawyer. I've never been able to confirm this, but I've always thought that was one of the reasons why she has unprecedented creative control over the Harry Potter property. The only part of the property that bleeds a little is Portkey Games, but even then, there's a ton of consistency in terms of how spells and potions work and world-building places and characters.
Jo will never sell. There is no amount of money large enough to offer her for the property; she already has enough money to support multiple generations of her family and her charities. Harry Potter is her baby and she will not risk it falling into the wrong hands. Warner Brothers attempted to wrest creative control from her when they wanted to start work on Cursed Child; she took back control again and worked on the story with the (well-regarded) playwright they hired, Jack Thorne. She wrote all the screenplays for Fantastic Beasts. At any point, she could have cut and run like George Lucas did, but she never has. I'm sure there's been offers, but it's not a question of a number for her. The only writer I know who had that kind of control over a property was Christopher Tolkien, and Amazon had to wait until he fucking died to get the rights to the literal appendices of Lord of the Rings, not even the actual Silmarillion or expanded universe. But that is why we only recently are seeing games, merch, more movies, and television shows.