I, like many others, am always slightly irked by Apple's refusal to let you select a default application. I think that if they allowed this, it could potentially significantly renew the appeal of iOS - it would move more towards being a platform, rather than a launcher for a collection of apps.
IMO that would represent a fundamental shift for Apple as a company, -really a change of character. My guess is that as Apple starts to lose marketshare (unless they can grab onto a new edge of innovation), they will make a decision to do something like this. But, because so many Apple users enjoy the walled garden they provide, I think it will hurt them more than help.
I suppose if they did it carefully I would agree. I guess what I am saying is that an Apple that made that shift isn't the same Apple anymore, and I wouldn't expect them to be able to open up and yet make sure their UI for the regular users was preserved. Many regular users might be lured into a hybrid experience they don't appreciate.