I think that joeblow7777 is spot on in his assessment. One group that would want that much storage would be those who do not trust the cloud, and want to keep literally everything on their phone. This group, however, is not large enough to trigger Apple or any other phone maker to put that much storage into a phone.
That being said, ultimately some manufacturer will put 512 into a phone, just to say they have done it, and the race will be on for everyone to do it because no one will want to have a backwater phone with less than 512 on it, even if they don't have anything to put in it.
The other thing is simply Apps becoming larger and larger and using up more space. Right now most of these use the cloud for the extra memory. I'm thinking here of apps that are heavy data users. The one way they can get their app to go faster is to have the data stored on the phone instead of having to upload and download from the cloud.