The tricky part is determining whether Borderlands is the exception, or the norm. If other studios try to follow suit, will the by ensured success? Or will they fall short? How significant is art style to the success of a franchise? I'd argue that Halo has some of the most bland sci-fi art in existence, but its console exclusivity and extremely fantastic gameplay mechanics more than made up for it, and set the world on fire.
CoD has terrible art, but the technical performance of the engine and positive feedback loop that keeps players coming back for a reward driven gameplay experience set that off substantially.
I'm not saying Fuse will be a success, just questioning the train of thought that it would be more successful if it had stayed the same, or that it will be less because of the change.
yeah it's hard to look at one series, but I know if you removed the art and humor from borderlands I would likely hate the game.
A ton of the realistic shooters that look like COD/Gears tend to fail as well, but again you would have to ask if that is a question of poor gameplay or lack of standing out... most of them they have horrid gameplay.
As for your last thing. Overstrike's art style helped it stand out. It wasn't so cartoony that I imagine many people were turned off from it. fuse on the other hand looks so blah.... I am actively turned off by how it looks and from the responses in this thread I doubt I am the only one, if Fuse's trailer had been shown off at E3 instead of Overstrike I doubt I would even be in here because I wouldn't care enough about the game to even discuss it (and i like the Resistance series). But figuring out if the change of style would effect sales is tough. With the change there is a good chance you lose most people who are sick and tired of realistic shooters and COD/GEARS clones. At the same time though you might get a lot of people who still only like those type of shooters. My views have always been though if you can't top the realistic gritty games in graphics top them in style, and right now Fuse is doing neither.