They aren't even necessarily done adding potter to the parks, and that's on top of the two existing areas. I think 2040 is perfectly reasonable for it to still be around.
What's in my head just now is that Universal are adding new attractions at a ridiculous speed. When you take a step back, it's incredible how much they're adding in such a short period of time. What's the plan going to be in 10 years when they might not have any 'old' rides to replace like you have now?
I'm finding it a bit difficult to see a ride remaining for 25 years at the current pace they're building at.