Yev here -> Interestingly, they aren't decreasing as quickly as they once were and that was part of the impetus for the increase. I touch on it in the article and we go further in detail in the original announcement, but it used to be that the data growth of the customer was more or less negated by drive capacity increasing and cost per gigabyte going down - but that trend has stymied recently.

Price per byte doesn’t really matter for an unlimited service. As that cost goes down, your users will have more and more data to store, in an amount that will basically cancel out.

SSD/HD ratio likely went up. Backing up a laptop ten years ago with 128GB SSD wasn't as hard a job as backing up 1TB SSD, which is now rather cheap.