Dropbox's own website claims they have >700M users.
18M paying customers is a lot (and I wouldn't be surprised if in terms of percentage of data stored paying customers are the majority), but nowhere near a vast majority.
If you were to look at the average cloud storage usage per user, split by free or paid, I think you would find that free users are using rather less space in total than you'd expect
But I agree, I did some of their cute ARG stuff for extra space when they were still acting like a startup and otherwise haven't paid them a cent.
Yeah except in this case this entire complaint is bullshit because somewhere there was user error. I tested it and it worked totally fine just using mobile browser. Simply offered to get app or continue using browser.