What do people do to need 256 GB on a mobile device? To store music, most of which they never listen to or only seldom do? Or to store videos, most of which they never watch or seldom do? This is what the cloud is for. Take music and videos away, and what usage needs more than 64GB?
For me, and it will differ for others, I don't need that much storage. I use my iPad for consumption - ebooks, browsing, texting, and doing lightweight email. I don't game, or edit videos, or do spreadsheets or create documents on it. I use my rMBP for those kinds of things.
I have an iPad3 which is now obsolete thanks to Apple. It has 64GB with 44GB free. I got a 10.5 inch IPP to replace it, but not for more storage. I honestly don't understand why 256GB would ever be warranted, much less 512GB. I've had systems with large HDDs, and never used more than a minimal amount of them. I've used Sharepoint or Notes teamrooms or some other networked storage for almost all of my files and data. Local storage was hardly used. Even large application environments booted out of SAN and had very little local storage. The iPad is a personal device, not a corporate client. But the thought is the same, and even more pronounced for personal usage - what is the need for so much local storage? I mean, if you want it, go for it. But what do you need it for?
I tend to think people are pack rats, too. They never garbage collect their unused files, and just let things pile up. How many restore points do PC users have on their systems? I bet almost no one ever cleans out the old ones. And all those moldy install files take up space, too, on Macs as well as PCs.
The new iPad is a wonderful device. IOS still limits it a lot, though, and I don't see IOS11 fixing that, even with all its glitzy new features. But the direction for storage is very clear to me, and that is toward personal cloud space. The vast majority of users will never be out of wifi range, so cloud will always be available to them. Personal devices like iPads are meant to be presentation clients, with enough local storage for transitory needs, and permanent storage will be elsewhere. So how much local storage is really needed for that? I suspect that even the 64GB on my new iPad is overkill.