Perhaps I should clarify what I meant to say.
1) The Wizarding World just opened months ago, which means a Phase II would not open for years. With the books finished and the film series to be fully released in less than a year, will a theme park area that takes up about a quarter of the size of Islands of Adventure be relevant for decades? It takes a strong franchise to justify the theme park attraction; how often does a single franchise gets its own theme park section?
2) Of the parts of the series that haven't been told in WWOHP "Phase I", do anything of them even justify a theme park attraction? I guess I originally meant to say "of what's left"? Forbidden Journey is a medley of Harry's life, of his adventures throughout the books. Universal wouldn't add a Phase II unless they could improve upon Phase I, and they already told all of their best stories in Phase I. If Phase II happens, it will be worse, or only a slight enhancement.
3) I never meant to say the HP was a weak franchise; it clearly isn't because it's exceedingly rare for a single franchise to be granted a theme park land. But look at what's been done before. Marvel Superhero Island has been there since the park's inception. Marvel Comics has thousands more characters than Potter has, yet at MSHI we only see the most popular. Dr. Seuss created the most popular children's characters of his time, and only the most popular have a presence. The Simpsons, America's longest running sitcom, got one ride. Add to the list E.T., Jaws, Jurassic Park, and many more.
4) Understand that few of the people that visit WWOHP are HP fans. You're lucky if the typical guest knows what a dementor is, let alone what The Leaky Cauldron or the Ministry of Magic are.
I think it was a great decision to add Potter as a theme park land, and the series has the cultural presence to justify a theme park section, but I don't think it has the presence to justify two. WWOHP is a masterpiece.