I don't think it devalues Disney's brand at all.
Outside of Pixar and Marvel the only big movie franchise Disney has is POTC.
Their non-Pixar animation is no where as successful as Pixar and their non-POTC movies are no where as successful as Marvel.
If anything the Disney brand was slowly dying without mega-franchises.
and Disney will keep well over a billion in profit from releasing Pixar-Marvel-Star-Wars movies every year. Thanks to dvd, tv rights, rides, toys, and other merchandise.
It's way better than relying on strictly new ''franchises'' like John Carter and Lone Ranger.
Slowly dying? Please. Disney would have been fine with or without Marvel or Lucasfilm, the reason they bought those companies is because they lacked a key demographic that those companies filled. They were too lazy to get the creative talent to fix the issues within the live action department. Smart from a business perspective, but lazy for live action Disney films. Just look at the list of turkeys they've produced since Pirates 1, it's not pretty reading.