I would consider myself to be pretty far from a "Grandma"-type user. Nonetheless my Backblaze account only uses about 300GB. I pay for my mom's Backblaze as well. No idea how much she uses, but almost certainly way less than that.

Most people's precious data isn't actually that large. Unless you're backing up large amounts of media (photographer, movie collector, whatever) it's not easy to fill even 1TB.