Steam users are used to this. You buy a game and then wait a day while it downloads. No big deal. Steam even pre-loads some games before they're released so that you can play them immediately when they're available.
On average I'd guess that people only buy 3-4 big name games a year which are over 1-2 GB so they probably won't bust their download cap. Also lot of steam games are Indie games which are usually 300mb-1Gb. When I was younger and couldn't afford games I would rent one or two every week, if everything were to go digital that download cap would be reached in a week or two.