I think where Assassin's Creed is concerned, and games like it, there's a interesting debate to be made on when a game stops merely having RPG elements (i.e. stats) and starts becoming an RPG, I don't know the answer myself but I'm disinclined to include Assassin's Creed in there yet.
As for what is and isn't mainstream, I consider something mainstream when even third-parties have a frame of reference where it's involved. Everyone can recognize a reference to Star Wars, or when it's tropes are being used or parodied, whether or not they've seen the movies but when THAC0 or Armor Class is used in a joke in a show like Futurama, at the time it was a inside joke to nerd-culture, now I'm not so sure, I think it's still borderline. But who am I?
AC are not RPGs because they lack both elements that constitute one: 1. Character customization 2. Story customization (choices that bear consequences)
Every western RPG can be classified as such using both these features. AC lacks both, as you don't customize your character, just use different weapons and apply higher damage modifiers, story is also linear.
This also implicates that JRPGs are not RPGs since they usually don't have any type of customization.
