With solid state storage you can cram in effectively arbitrarily large amounts of storage in a tiny amount of space, so that will not be a problem with a thinner laptop. But it doesn't matter. Battery capacity will be lower, which means it won't be a professional laptop, which means it won't really need terrabytes of storage.
It sounds like the next MacBook Pro will basically be a better and slightly larger MacBook Air. That could be a smart short-term move by Apple, but it will kill them in the long run when their professional users jump ship and buy computers from Dell.
I'm glad Dell is still serious about building pro segment laptops. Samsung is probably going to want a bigger piece of the action too if Apple moves out of the pro space.