So your answer to a perfectly fitting question is "It would not work because other people might be suckered into choosing it?"
Hang on a tic.
Did you really just type something to that effect? What exactly gives you the right to make a blanket decision based on what you think other people should choose? What exactly is the issue here, that other people might miss out on all the bittersweet? I'm not quite sure I understand.
What players of this series choose as their preferred ending is hardly something that should be knowingly suppressed. After all, it isn't as though a "sunshine and bunnies" ending somehow violates the fundamental presentation of this series. We've had fairly perfunctory conclusions for the previous two games, why should we suddenly be afraid of one this time around? Because some pseudo-philosophical "intellectual" believes that we all must partake in equal suffering for the greater good of some "higher message?" No thank you.
Ask the community at large whether they would be content with a happy ending, and I suspect you would find at least half or greater in agreement with the suggestion. That alone makes the inclusion of an appropriately executed "happy" ending a no-brainer.
Since your arguement hinges on the notion that a happy ending would remove the impact from any bittersweet one, I have a simple response for you -- it doesn't matter. Why doesn't it matter? For the same reason that it didn't matter in Mass Effect 2; this is a video game. Video games are, fundamentally, designed to be won in some fashion. For many, "winning" is achieved by a tangible sense of victory. Bittersweet can convey that feeling for some, but the number will quite obviously be smaller.
Furthermore, "bittersweet" is a very polite description of the current endings. To me, I would probably choose something more along the lines of "dark and depressing". From lurking on the BSN, this was not the case for everyone. But an overwhelming majority of vocal fans did not particularly view them as such.
My main point here is thus:
You, as an individual, are not in a position to make an argument such as you have made. It is true, in the sense that a happy ending would be chosen far more often than any of the current options, but that is neither here nor there. The existence of a happy ending does not remove the "bittersweet" endings from the game, it simply offers another choice. This series is predicated upon choice. A large part of the community has expressed a desire for a greater sense of choice. A forced dark ending works just fine when it is appropriate in the context of the game where it appears (see Red Dead Redemption). It simply did not work here, not as it is.
If you, and others like you, feel that a bittersweet ending is totally fine, that's just dandy. You have three options at the moment that perfectly convey doom and gloom. You even have the right to continue choosing them upon subsequent playthroughs. There are some of us who would never choose such endings if there were other options. Fortunately, our preferences are just as important as yours.