Hard to say, because we never saw what they did with it. Any ending were they try to say the Space Nazis weren't so wrong is nonsense. This version of the ending might at least have been tonally consistent with the series.
Now, what if they were the radical space environmentalists to the parable of global warming?
It wasn't set up or even elaborated this way after the fact, but the Dark Energy thing could have justified itself were Dark Energy buildup something that could be reset by the cycles, rather than cumulatively build over them. If Dark Energy is something that is a seemingly inevitable part of galactic civilization in the ME universe, making such civilization tidy and easier to clean up would make sense.
The Reapers would rest on claims of technological determinism and self-interest. We know that e-zero (the stuff that makes the mass effect and dark energy) is both naturally occurring and artificially creatable, so it's only a matter of time before a civilization finds the Mass Effect. From there, Mass Effect just needs to be the best tech in the universe- either on its own or in conjunction with other tech paths- to make it's widespread adoption nearly inevitable. Mass Effect fields are a competitive advantage, and the civilizations that avoid it are weaker and fall to the groups that do. Even if/when civilizations start to understand the consequences- when stars like Haestrum start to destabilize- they're still caught in competitive traps where everyone has an incentive to still use e-zero even if everyone's stars will go out if everyone does. A galactic-scale prisoner's delimma.
So instead of 'genocidal war is inevitable,' the Reapers would have a slighly more familiar motive of 'selfish people destroy their world(s).' It'd still be on the cynical end of the spectrum of 'Malthus was right'- but that could also be what the Crucible is for. Something where beating the Reapers is incidental, not the real point.
Getting into real personal musings here, but if Dark Energy had been a thing I wouldn't have wanted the Crucible to destroy or control the Reapers- it'd be to destroy all element zero or trigger that dark energy tipping point right now, with the third option being a shakey cease-fire/standoff with the Reapers.
In option 1, Destroy, instead of all synthetics what you'd be doing is using all that dark energy buildup to destroy/render inert all element zero in the galaxy. More will come eventually- either naturally or artificially- so the problem is just kicked long, long into the future or the same delimma- but for now all e-zero is inert. That kills the Reapers, obviously- but it also cuts the knees of galactic civilization built on Reaper (e-zero) tech and space travel. There'd be other tech paths possible- a possible loot from a kept Collector Base- but none as good. An ending with Renegade themes, and in which the Renegade decision to keep the base may leave someone (albeit Cerberus and/or the Alliance after Cerberus station) with the Collector Base technologies that could accelerate the rebuilding afterwards.
In option 2, Accelerate, we don't necessarily destroy the Reapers as deny them their goal. On top of damaging them for some period, we tip off the galactic tipping point, giving our galaxy another, oh, thousand years before the stars start going out. By giving them nothing left to save, the Reapers have little reason to stay and finish the harvest, and so they flee the galaxy. The Council system survives, Relays and all, but with an ominous future. The time left is limited, the Council system must come together to manage the galactic exodus, and the Reapers are still out there and may try to keep us from reaching other galaxies and doing the same there. But with time to prepare and all of us standing together (or not), Shepard gets to live out their life and the Paragon-themed endings suggest species come together to organize the Exodus in the future.
And in option 3, stand down, we don't activate the Crucible but threaten the Reapers with it if they don't stand down first. Requires high reputation, strong war assets, etc.- but since either option ruins the Reapers, we can hit them with MAD. Cue epic stand off/stare down with Harbinger, the threat of 'I really will do it,' and... a lot of uncertainty. The Reapers stand down, and fracture along with what should happen. Some no doubt conspire to indoctrinate and worm their way in to deactivate the Crucible to resume the harvest. Some go off to build their armies for that day. Some just leave on their own, mysterious reasons. And some try to work on solutions with the new galaxy- which is just as unstable. The wartime alliance of necessity will break apart, the tension of the moment keeping everyone together will fade, and whether we're united or not there's still the question of who and how the cutback of e-zero usage will go. It's a very short-term solution with no clear direction. Maybe a solution will be found. Maybe it's impossible. Maybe the Crucible will be used again in the future after a Reaper attempt. Maybe the Reapers will successfully sabotage it and resume the harvest. No one knows. But everyone still alive gets to live, galactic civilization goes on, and Shepard walks away... for now.