The average person on the 50GB Dropbox tier might only actually have 25GB of stuff. Now they're on the 100GB tier, but they still only have 25GB of stuff.
This is almost certainly why they also don't offer smaller paid accounts (like 10-20GB) -- they know users of such tiers will probably keep their accounts maxed out. I know I almost always utilize nearly 100% of my 2GB free account, and I'd probably do the same with 10GB.
The average person on the 50GB Dropbox tier might only actually have 25GB of stuff. Now they're on the 100GB tier, but they still only have 25GB of stuff.