If your theory is "they did it to sell dlc", then there are lots of people who have already thought about it.
Even I was suprised how bad the endings were. I knew it would be a DEM, I knew from the script it would be stupid, and make me think even less of the reapers. But what really suprised me, was the lack of impact your choices have.
Not even a damn epilouge. They didnt even try to pretend your choices had huge impact.
No, I mean a theory about what the ending means and where the story is leading, not what BioWare's business strategy is. I think there is more end content coming, a Truth DLC or something. I do think that if BioWare charges for it, there's no defending them. Charging people for a proper ending would ruin their reputation, they'd deserve every bit of backlash they'd get. But there are rumors that a Truth DLC would be free. Maybe they'd even incorporate it into a patch. I'd have no problem with that; it would just mean we had to wait a little bit to get the proper ending. Not the end of the world.
I find it very hard to believe that BioWare would write the first 99.9% of the story so superbly, then completely botch the (arguably) most important part, the ending. I also find it hard to believe that they think the current ending is satisfying. All the storytelling up to that point suggests that they're smarter than that. That it doesn't gel at all with their hype prior to release ("closure, no loose ends, different endings for everyone") just gives me more reason to believe we haven't seen the whole story. In fact the hype might have been preparing us for the ending we got, cautioning us to be intrigued by and suspicious of the ending rather than accepting it for what it is and flipping out.