I took my Harry Potter mad 11yo son to the WB studio tour in March for his birthday. It was ridiculously busy on just a random weekday (lots of school trips) and it was hard to get tickets, they are sold out a few months in advance. I don't think a theme park would take much business from the studios, HP fans are going to want to see both - especially if they're travelling from USA or Europe, or even northern England.
I think there might be a game of negotiations going on behind the scenes here. WB want too much money from Universal for the rights, so Universal have said forget it we'll be successful without HP anyway. hoping WB will reduce their price when they see Universal planning to go ahead without HP. Universal might be secretly thinking if WB drop their price they may include HP afterall. Having HP as an IP would make them all so much money and make the fans so happy it would be mad not to.
*Normal disclaimer of I'm sure it will be a great park whatever, and if they didn't have HP they'd be sure to have other creative, original lands to make up for it. But I really want HP personally - I'd get a season pass for sure. Florida is out of reach financially for my family for now so its not as simple as just go to Florida for HP.