1. Valve is going after a market that is already used to all games being digital. There won't be any backlash this time as it is not removing features (trading, lending, etc) that people were used to having. Plus people will expect the cheap prices to continue, as opposed to console digital games that have always been more expensive then a physical copy.
To your points:
1. Agreed, no one is expecting anything else from this.
2. As a steam user, I have never run into the $10 AAA title. Maybe 2 years old stuff is that way, but then only if it sucked to begin with.
3. This is mostly correct, but that actually would support the idea that the Steam boxes will fail. Most users don't want to buy a special box/pc to play games, they use what they already have. So, why would they bother to buy a special box that is more complicated to learn and won't have all the same stuff as a regular PC?
4. Steam is well established, but only with people outside the casual crowd. Most casual gamers are more interested in IOS or Android apps. Still free and very cheap.