The core fallacy in your argument and the fallacy inherent in the overall "games as art" belief is that an an audience's participation in a finished product no longer makes it art. You can make an immaculately sculpted wooden chess set that is a work of art but the actual game of chess to be played on it is not art. Sure, a group of people can make a work of art like a film or a mural, but that's not Mass Effect.
Allow me to paraphrase what Garnett Lee said last week: Mass Effect tells you what parts of the mural you have to fill with color and the only choice you have is what color of blue you want to use. You can use baby blue, royal blue, navy blue, etc. Whatever you pick, that part of the mural will be blue.
This isn't the audience "participating". You're going down set paths. No single player has a storyline that BioWare didn't anticipate. If this were the case and if Hudson's PRspeak were to be taken literally, then choices I made in ME1 and ME2 may have caused a butterfly effect to have the final battle take place on, say, Illium instead of Earth. But Mass Effect doesn't work that way and I'm fine with that.
But people who believe the so-called "choices" they made gives them a say in how the series should end, you have a sense of entitlement that you never deserved in the first place.