I have a feeling it's going to go something like "Her role of being the sponge for all the world's "evils" isn't actually necessary" kind of thing. Like she's sponged it all to protect humanity from having to deal with it, under the belief that without her doing that, the "evil" would bring an end to the world... but it'll actually be the case that when she's persuaded to abandon her "duty" and let the "evil" out, instead each living thing on the earth will bear a bit of it, and that'll be across so many beings that it'll actually be easily managed. I kind of forgot, but was it ever made clear that it was specifically necessary that someone had her role? Wasn't it that the god appointed someone with her role because the god couldn't/didn't want to deal with it on their own. If so, there's no need for the singular sacrifice, it's just the necessity everyone believed...
Or, you know, the fact that people that look like MMC and FMC are in the "story" of the god and their appointed "janitor" except MMC's proxy seems to be the janitor, and FMC is the lover is indicative that being the "janitor's" lover is the actual intended role of the "sacrifice" that the MMC was to be at the start, to free the current janitor by taking their place, and it'll be that tragic ending instead, where the MC's just trade places instead...