I am not the first one to come up with this idea, and there is a clear trend of "prophecies being wrong" so I wanted to sit down and throw some of my own ideas into the pile. Mainly, I think that Death Devil will actually end up the big good, with Denji/Chainsaw Devil being the big bad. I think that is the part of the prophecy that everyone get wrong: "Age of Devils" is a good thing (somehow) while "Age of Man" is opposite. That is why it is important, I think, that Makima mentioned that Chainsaw ate other "afterlife options". I think Death Devil simply wishes to "restore the status quo" and return things back to normal, because due to other afterlives being "gone", Death is now "flooded". Potentially, Chainsaw might be evil because he wishes to erase death as a concept as a whole, to never "die" and thus "never lose itself", which would feed into the narrative of the story, which is "when you try to control things, it only makes it worse; freedom is good."

I and I raise you one on how Death Devil might look; instead of goth mommy milkers, goth gilf. Just an old granny around whom death just happens - they are not evil and actively go out of their way to cause it, it just happens, which is why they might live as a hermit and "out of public's eye".

Long story short, my main idea is that Death Devil is good because she understood the nature of life, which is, that controlling things is bad and best approach is to let life flow. Once Chainsaw Devil fucked it up, Death Devil now wishes to "bring about the Age of Devils", which literally means getting her "afterlife friends" out of Chainsaw. Age of Devils, then, might just be a name for "things going back to how they were".

But it would be funny if Death Devil is so far removed from humanity, that she doesn't even look anything like a person; like, they are just a fucking sapient cube or some shit, completely unhinged. Like, if a homie just needs to be in your presence for shit to get fucked, I doubt that physically strong body would be required. Hell, like Hell Devil, Death Devil might also be flat-out immune to "dying", similar to other afterlife devils, which is why Chainsaw's vore is the only actual way to "eliminate" them.

Edit: Fun thought, but what if Death Devil isn't the strongest one? I mean, technically, it is possible to not fear death - but what about something like Paradox Devil? Devil that feeds on the fear of the unknown (which is even more fundamental than fear of death, I think) Hell, Chainsaw Devil might be the "Nothigness Devil", given his abilities.

tldr; "Age of Man" is actually a bad thing, humans are real monsters, "Age of Devils" is the natural order of things, Chainsaw Devil is actually Oblivion/Nothingness Devil while Death Devil is the good guy trying to get things back on track.