True, but hard drives wear out and need to be replaced. And the type of technology you use keeps getting better, and different. Right now, SSD are more expensive than the old-fashioned kind, so the amount of space is limited.
Here's the thing, though: the amount of space was always limited by the physical size of the drive. There really is no such thing as "unlimited". If you run into space limits, you might want to consider using a CDN of some sort. I currently get 10GB a year for only $20, and use it for media files, which keeps my DH footprint small, and (possibly) speeds up my site a bit, since those files are accessed only irregularly and don't get in the way of regularly accessed files. (I have no documentation for this; it just seems like a sound idea.)