For me, it's because rewrite endings in other media, especially based on audience response, has a fairly terrible track record. Classic example being the ending of Blade Runner, which was made dramatically shittier because test audiences wanted a happier, less ambiguous ending. I don't want to get into the ME3 stuff again specifically, but the whole idea of people rewriting fiction to suit the preferences of a mass audience really rubs me the wrong way. To the extent that this stuff is art, I feel we should respect the role of the artist vis-a-vis the audience. I also didn't mind the ending and didn't think the ME story was any great shakes to begin with, so that impacts my opinion too.