As a concept though, Dark Energy is just as flawed.Creative projects is a problem argument here even the entire writing and dev team were unsure how the concept would play out, and the ideas presented in the end, while attached since the beginning of the series, are either too vague or to fundamentally different from what were seeing.
For example, Sovereign says the following "I am beyond your comprehension, I am Soverign." ,"Organic life is nothing but a genetic mutation, an accident. Your lives are measured in years and decades, you wither and die. " and "Organic life evolves, advances, and at the apex of their glory they are extinguished."
The entire exchange with Sovereign would contradict Dark Energy as a theory for three reasons. One, even if the Reapers are attempting to control life and order it around as much as possible, then what motivation is there truly, outside of "we represent order" that they have to do this? If the full-blown destruction of the galaxy was possible through Dark Energy, as Drew states, then the Reapers logic of allowing organic life to exist makes no sense, especially through a machine.
Two, if they are immortal beings, the need of Organic life then makes little sense to the Reapers logic.Wiping out and cultivating races and civilizations every 50,000 years so that they evolve to use Biotic powers that can destroy the universe also contradicts the logic of the story-thread;I.E, the fact that the Reapers simply wipe everything out to prevent the destruction of the Galaxy, ergo saving themselves from destruction, is a glaring inconsistency.
As is the fact that the reapers also built the Mass Effect relays, according to Soverign again "Your civilization is based on the technology of the Mass Relays, our technology." Why would they create technology that would also hasten the destruction of the universe through Dark Energy?
The only explanation that possibly can work comes from Drew himself. "Then we thought, let's take it to the next level. Maybe the Reapers are looking at a way to stop this. Maybe there's an inevitable descent into the opposite of the Big Bang (the Big Crunch) and the Reapers realise that the only way they can stop it is by using biotics, but since they can't use biotics they have to keep rebuilding society - as they try and find the perfect group to use biotics for this purpose. The asari were close but they weren't quite right, the Protheans were close as well."
But it is still flawed, because then the time clock for destruction makes little sense; every 50,000 years for mellenia this has been going on, and the timeframe of this big crunch is very unclear, even in Mass Effect 2.
So the truth of the matter is that Dark Energy is a flawed premise in of itself. Part of that creative process you mention is likely why it was not used. As Drew states "I find it funny that fans end up hearing a couple things they like about it and in their minds they add in all the details they specifically want," he explained. "It's like vapourware - vapourware is always perfect, anytime someone talks about the new greatest game. It's perfect until it comes out."
Something else that might be relevant to point out is that the decision of the Reaper motivations, Dark Energy or otherwise, weren't finalized before or even after ME1- it was almost certainly made post-ME2. Which means that any discussion about Dark Energy really can't be talked about in terms of 'well the whole trilogy would be different', but really on how ME3 alone would be changed- which makes ME2 and ME1 judgement points.
We know from the dev interviews that the devs hadn't decided the Dark Energy plot from the start- Drew's comments of it being one of the ideas thrown around, but that it wasn't The Key Plan- but we can also see elements in ME2 that were clearly left for ME3 to use as the springboard if that's what they wanted to go with. Haestrum, and what's-her-name, the Noveria lady who mentions her company looking into Dark Energy.
ME2 gave a couple different angles that ME3 could have used, each being a retroactive foreshadowing: Reapers could be organic-synthetics (Geth-Quarians, Collectors/Smoothies), Reapers could be dark energy (Haestrum), Reapers could be peace-makers bringing order to chaos (the looming Batarian Rebellion, the other pending wars, the implied utopian unity of ascending to Reaperhood), or the Reapers could be preservationists saving us from ourselves (the Drell, Quarians, and Krogan as near-extinction species). ME2 gave us all of these... but some of them really don't make sense if there was already a plan for which one they wanted. Haestrum doesn't make sense in the context of Organic-Synthetic... but Organic-Synthetic, and most the other B plots, don't advance Dark Energy either.
But, considering what we know of the Devs and their planning (their own admission of not having a clear idea, ME2 Cerberus was a post-ME1 invention, the unexpected stumbling over the foreseeable consequences of the Suicide Mission making the entire cast killable and plot-non-critical), it's really, really likely they just didn't make a decision until ME3 was next on the board.
What does that mean about discussing Dark Energy?
When you discuss Dark Energy, you should not only take the Devs on their word- about what it was (cumulative Dark Energy usage, that it was never the original goal)- but the real vaporware start point of 'the series would be changed/improved' would start at the decision point of... after ME2.
Meaning only ME3 would actually stand to be changed if they had gone Dark Energy... and at which point, we might as well ask how much would have been changed? Considering how little synthetic-vs-organic mattered in other ME3 decisions- the choice of the Crucible as a not-understood 'I Win' device, the narrative arcs, character missions- the relevance of Dark Energy as the motivation choice really needs to be considered conservatively.
I mean, sure, you could argue that Dark Energy would have meant that Bioware would have gone back and changed and better planned the series from the start... but if you're going back to fix ME1 and ME2 to make them work with Dark Energy, why couldn't Bioware fix them to work better with Organic-Synthetic?
For a meaninful discussion of Dark Energy theory's applicability to the series, keep the changes to ME3.
We know from interviews that ME2's plot wasn't even decided pre-ME1 because Cerberus (a key driver of the plot) wasn't even intended to be significant in ME1. And IIRC, the leaked script for ME3 dabbled with Reaper motivations- so that