Here's my thought on the whole use of the "it's art" defense from someone who is an actual artist, both a 2d and 3d artists, and someone who used to write fiction a lot and was trained to do so.
Great art is not created in a vacuum. Even the greatest of artists had peers and predecessors to with whom to compare, critique, and learn from. Leonardo DaVinci had the works of great painters, scientists, and others to call on before him even though he is considered one of the greatest pioneers of art. Then, especially in story, the greatest writers actually always have peer groups to discuss and learn from. That is actually the creative process, and part of that process is for the creator to create a quality product that fits. One does not paint a 3 panel painting with the third panel a mess, or maybe someone does, but there needs to be a reason. Otherwise, such artists, while respected, can be rightfully critiqued or ridiculed. And yes, I've seen artists go and repaint something and make it better from critique, even after technically finishing the product.
Most people actually don't realize how much of the creative process is revisions in response to critique, and a lot of that actually comes after the artist thinks he's done. And when that artist changes things, he's doing it for various reasons, but most artists who make those changes aren't doing it to appease the masses, rather, it's because his peers have shown him where there are problems in his work. After all, I think all of us have had plenty of situations where some idea we think a world of cool, turns out to be a turd the moment we think on it, or tell it to someone else.
Many instructors and peers of mine have said that the true strength in creating art, whether that's painting, writing, or music is the ability to take criticisms well and make your work better from it. It's not just to become some wall that considers your work godlike.
As a fellow creator, who knows how stories are told, I look at all the things they did right in ME1, ME2, and 99% of ME3 and see the ending as a breakdown of that process. Something went terribly wrong, and there's a real reason why the community has reacted as it did. I mean, anyone ever seen Project Runway?
ME3 and the series is as corporate as you can get of a product, I laugh at anyone trying to pass it off as purely art. It is still art, and it is definitely art to a great degree, but to try and say every part of it is art and worthy of some higher level protection is pretty much holding up a paper shield trying to deflect just and proper criticism (from most people anyway). A product can be both art and a product.
After all, Leonardo, Raphael, and even many modern painters today are painting under contract, or with the intent to sell their painting, and they demanded quality. Even Van Gogh, a failure of his time, still painted to make money. His work was genious for its originality that made his pieces stand the test of time. This ending isn't genious.
Oh, and artistic vision in a corporate product where they tie multiplayer that has microtransactions deliberately into single player via an arbitrary percentage number is a joke. I actually thought the idea was cool at first when I first heard it, contrary to most, but the actual implementation was definitely a money grab. The same goes for the DLC story elements, which although I'm somewhat okay with, still makes a mockery of these artistic protections people are spouting for the game. Again, it's a corporate product, still a noble and worthy artistic production.
It's clear that there were many very non-artistic decisions made in the creation of this product. This corporate input clearly affected the game in many ways. Why shouldn't input from the people that matter?