I don't think demand would stay low if other sites started doing the same thing.

Companies basically have to advertise, and they'll buy time with whatever outlet serves the eyes they want to reach. Ford Motors isn't gonna let MyPillow.com take over ESPN.com on Super Bowl day just because they don't like the advertising provider, method, or architecture.

You're right, but it's impossible to overstate how uncooperative publishers are. They refuse to work with each other and every major publisher-run ad exchange has failed. Maybe privacy and data regulations will finally push them to change.