I think you're being intentionally and wildly naive. You think with the reviews she and the show got, and the almost complete snub from the Tony Awards, including for one of the most Tony-level roles there ever was ... they think they made the right choice?
Do you think it's possible that the fame of the title... due to the original show, the acclaimed and loved movie, the star of both and her global stardom spanning 6 decades and her constant association with that title... sells a pretty good chunk of tickets on its own for awhile, and then the fans of Beanie and Jane account each for a chunk ... but also it hasn't been running that long. Do you think the producers are confident it'll still be selling well after 15 weeks more of audiences have had their chance to buy tickets?
I think whether they think they made the right casting choice or not... they will be quite eager to see what will happen with another Fanny. If that's even in the cards, I guess it'll depend on how it's selling by August, and if they can get anyone with a box office name to sign on.