I can see why they ditched the dark energy ending. I mean it's an almost existential threat and I can't see many players going along with the reapers. I do agree that it's interesting that their motivations aren't totally selfish.
Yeah, it's not a great ending, but the benefit of it is that it doesn't contradict series lore and it was foreshadowed in the previous game. The current ending wasn't even foreshadowed properly in its own game, so in a choice between an ending that vaguely works and one that doesn't work at all, it's confusing why they chose the latter.
Personally, I don't think the Reapers needed an explanation beyond being giant machines that see themselves as being above us and try to impose order on the galaxy. Humans don't mow the lawn for a greater purpose, we do it because we don't like how unruly grass gets when it grows too long, and we don't care how grass feels about it because it's clearly "inferior". That sort of explanation would have been fine for me