Doesn't work. People would just create dummy accounts with 2GB folders, share them with themselves, and have infinite storage.
If they offered a 100GB plan for $20 / month, it would destroy their $125 / year / person plan since I would imagine that in almost all cases that's a much cheaper arrangement for teams.
That's why the current solution is such a disaster. The alternatives are either too easily gamed or they rape Dropbox's revenue.