Your example re. <1080p> avatar would use up a lot of people's bandwidth for the month. This is chicken and egg, Apple are merely making headway preparing the plate to serve your omelette. And when the omelette turns into roast chicken, they'll have a bigger plate ready and waiting for release.
So what changes in the future? Does bandwidth magically expand? Because the owners of the bandwidth are not going to cut profits to install major upgrades when they are the only show in town (or one of two shows, and the other matches their price, sells video subscriptions, and also has little to no incentive to expand bandwidth).
Get 1080p

TVs into lots of homes. As the numbers grow, studios will be seduced to test 1080i/p content in the iTunes store. No 1080p

TVs in homes means NO seduction. We've had 720p for 4 years now. The world of video has moved on. Hopefully, Apple will too.
Else, they'll sell less

TV units, only to those who accept "720p is good enough", "until there is content in the iTunes store", "until nation-wide bandwidth is upgraded for 1080p", etc. All excuses, begging to fuel sales of other solutions for those desiring a true head-to-head challenge for BD, satt/cable set top boxes, HD Tivo, WD HD boxes, and so on.
Until the bandwidth owners feel the pressure, they don't "waste" money on nation-wide infrastructure expansion. Recall how AT&T's first reaction to being called out on limited 3G coverage by Verizon was to try to sue Verizon into stopping (the ads)- not sink a lot of cash into massively expanding their 3G bandwidth. Even now, they run their "97% coverage" commercials, not mentioning that that coverage includes a lot of 2G "edge" instead of 3G. And aren't these some of the very same companies in charge of many people's broadband connection (along with other companies that have a lot to lose if the public finds a good alternative to their lucrative cableTV plans)?