Originally posted by Nemesis X
The ending still felt contradictory to some stuff though IMO and from what I understand, the ending we got in ME3 was but Hudson's Plan B as the deadline approached and there wasn't time to do the original which had something to do with the red star you see by the planet Haestrom where you recruit Tali in ME2 IIRC.
Karpyshyn himself has said that there never really was an ending planned in stone. The dark energy ending, that you are referencing, was one idea they had, but he said it was never really fleshed out beyond a very basic idea.
From what I understand they changed it not because of a deadline, but for other reasons. An early script for ME3 leaked several months before release, and included the dark energy ending, so many think that they changed the ending to keep it a surprise, though it's possible the change happened prior to the leak, since the leaked script was an early draft. They also have said that some of the writers supported the idea of the ending having to do with the theme of synthetic vs organics, as that is a theme that was much more prevalent throughout the series, where the dark energy one is kind of out of left field and, besides relating to the Haestrom mission, isn't very relevant to any themes explored in the series.
There are arguments to be made for both. I kind of think the theme they went with was the better choice, as I agree with having an ending be relevant to a major theme throughout the entire series. However, I think if you just look at the choices involved and nothing more, I do think the dark energy choice would have offered a more interesting choice. I think it's a case of 'the grass is always greener'. The synthetic vs organic ending has its flaws, but the dark energy one would have had its own flaws as well. There probably would have been complaints towards it, too.
Personally, I think the best possible choice would have been to keep their motivations unknown and mysterious. Maybe offer some vague hints and little more. We never really needed to know their motivations to know we needed to stop them, and I think the Reapers were most interesting in ME 1 when they were portrayed as a totally mysterious entity telling us that we can't possibly comprehend their reasons for doing what they do. This ending probably would have also upset a lot of people who desire to know as much as possible and don't like having questions unanswered, but I would have liked it.