We'll have to agree to disagree then because I see no indication Maxime even has an "old self" to lose. She's completely, 100% obsessed with the MC, to the point she has literally no agenda beyond being with him. Their relationship is painfully shallow - the game cut away during their dinner date because there was evidently nothing worth seeing the two discuss.
More than anything, Maxime reminds me of Heather, the ghoul the player could inadvertently create and enthrall in Vampire: The Masquerade Bloodlines. Both were junkies completely hooked on the player character's 'essence,' but whereas VtMB treated Heather as a side character and morality pet, GoH insists Maxime is a full-on Love Interest. That just doesn't work for me. If Maxime had a life that extended beyond the MC's dick, or if we'd spent meaningful time seeing her and the MC interact as friends with mutual interests rather than just fuck buddies, maybe I could buy her as a LI; I'd certainly be much more interested in her content. But we didn't, and so I have no desire to watch her drool over the MC for the Nth time.
With the caveat that I do like her character -- and her scenes -- a lot ... I don't actually view Maxime as a "love interest" in the context of this story.
Could be that I'm just overthinking things (wouldn't be the first time), but my sense is that she primarily serves as little more than a plot device. As you mention, there's no "conquest" involved with her -- all those steps have been taken before we start the game. Rather, I think her presence is two-fold:
1) She acts as a "bridge" between his old, (mostly) human life and his new-found demonic identity, and
2) Exactly as Astofiel says -- the fact that she's already so close to "worship" status significantly speeds up MC's development. Broadly speaking, it's plausible insofar as it relates to the plot and it avoids having to further drag out the process for us as players. Plus, as we saw at the hotel, she can "help" with bringing others on board.
Right now, I think there is only one true LI, in the sense that we have an opportunity to build a legitimate relationship with them, and that's Julia. I think Brie becomes one sooner rather than later, but that's it. I'm undecided on Ruby so far, but I'm leaning more toward the "plot device" category for her as well.
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