Well, they could certainly go further with potter. They've only briefly touched up on some of the major areas and activities in potter in the current rides and attractions. From forbidden forest to the actual castle grounds, quidditch, the lake, the Triwizard tournament, the other schools, there's plenty of things that can be expanded upon. Are we sure the remnants of the lost continent are earmarked for a different franchise, at least all of it?
Also, I'm not a huge potter nerd. But, I think it is almost safe to say it has stood the test of time. It's been over 20 years now... It's a modern day Star Wars with it's cultural relevance and how it's being passed down from those who were kids and teens when it came out, down to their children. Its a book series that has outsold LOTR 3-1. The games, the merchandise, the movies... The middle age folk donning capes posing with wands in the middle of Diagon. It's a cultural phenomenon. Potter nerds are the new Star Wars nerds, and the drip of content from that universe is perfect. Not too much, not too little, even with a few bad movies, The works maintain their "gospel". Personally, HP kind of reflects on a cultural level how star wars was passed on to me. I don't know how anyone can doubt it's relevance. Even with J.K saying... Things. And even with the Fantastic Beast snafu, there is still much demand for stories from the world, and there's a demand for people willing to pay to play in it. I think handled properly, the tv series doing work to disseminate all the sub plots in the book and translating them onto tv in the age of streaming is only going to increase its longevity.
As for the contracts, I doubt there's a clause or language there that would allow for either party to say "guess what, Potter is leaving universal and now belongs to Six-Cedar flag fair".
In the realm of theme parks, potter changed the whole game. Let's not forget what we're talking about here... From an underdog to contender, the work Universal put into potter changed the whole game at the high stakes table, Twice...And will do so a third time.