Digital storage has always been increasing. Remember floppy disks? They held a whole megabyte. I remember the first time I got a 1GB hard drive, thinking, "When would I ever need that much space?!" And yet here we are, where 16GB on an iPhone is no longer enough, just as that floppy disk is just a drop in the bucket compared to today.
You see, storage has increased in part because the file size of our documents and applications have increased. Back in the day, you could run a whole game off a floppy disk on your Apple II or Commodore 64. But as games have increased in size, they require more space. You certainly couldn't put Skyrim on a floppy disk. So, as time goes on, to keep the user experience a good one, manufacturers have been upping the capacity of hard drives and the like, simply in order to retain the bare minimum of customers' data. But with how inexpensive storage is these days, most companies haven't taken issue with providing customers with ample extra room.
Digital storage largely hasn't budged in the last couple of years. 500GB has more or less been the standard on laptops for at least 5 years or so. Sure, the upper limit is always going up, but it otherwise hasn't changed.
Storage spaces increased very quickly in the early years simply because the technology wasn't there yet; it had nothing to do with applications getting bigger. Like, Windows 95 need 13 floppy disks.
If anything, average storage spaces are getting smaller as we switch from HDD's to the more expensive SSD's.
Working in technical support, I can tell you that people are running out of storage on their iPhones constantly.
But isn't this bias? The only people who are going to come to you are those with problems.
will inevitably be recording 4K video when/if it appears on future models.
No, they won't. Because if 60FPS recording on the iPhone 6 is any indication, 4K recording will have to be switched on in the settings and will inevitably come with a warning that it takes up more space. Unless the user deliberately goes out of their way to enable 4K, the average user is still going to be shooting in 1080p.