Well, I suspect the majority of Dropbox users store well under 1TB. As it is a limit few will actually be pinned at the limit either. Who knows what the average utilisation is, but I would wager many barely scratch a few gigs.
Moreover, if you have spare local capacity, farming it out is only a bandwidth/power cost - the storage device is just sitting under utilised without using its full (80%) capacity. So, the maths here is different too and I would also wager that most people don’t use even 20% of their storage device currently.
Take both of the above into consideration and the figures start to look a little more compelling.