Mummy isn't closing for Secret Life of Pets. That ride is slated for a currently empty space, the former Soundstage 28, next to Transformers. Mummy is still likely to close (along with surrounding working soundstages) to accommodate a Diagon Alley expansion for 2018/19 to counterprogram Star Wars Land.
While $115 is certainly more than I feel USH is worth, just like Disney, people will pay it. The allure of Potter shouldn't be underestimated, especially in the ravenous SoCal market where millennials love theme parks. USH is very confident, and they have reason to be.
I don't think any theme park is worth $115 for one day. I'd blanch at paying that much for a day in Tokyo DisneySea.
The problem I have with Disney is not how much they charge, it's their tone-deaf PR explanations for it. "We're trying to manage crowds." If that were true, the new parking structure would have opened in 2012 like it was supposed to and the resort wouldn't be lacking for substantive additions in the last 4 years (parades and fireworks don't count). They need more capacity in both parks and despite seeing the storm of crowds growing and growing, they sat on their hands and just started charging more.
USH, as far as I'm aware, has made no pretense about spreading crowds out or some other faux-altruistic BS. They're charging what people will pay. That's what Disney is doing too, only Disney pretends they're doing you a favor by doing it, which makes it slimier.