I haven't read through the entire thread, but I think the biggest problem is suddenly forcing it on every already existing game existing games. Retroactive changes like these have a tendency to upset people.
The idea itself is excellent.. I can develop plugins for Visual Studio and charge for them, why not game mods?
At the extreme it's no different than developing a game that uses the graphics driver.
If they had just made it optional for developers on new titles (only) it would have been a whole different thing, and no one would've had the energy to be upset. It would start with a couple of new games that would become popular with modders with a dream to make money from it, which would make those games get better mods, and the $99 ingame cupcakes would be a funny joke instead of the general opinion of the entire system.
And they did realize that: