If her ego is dead then what remains of Reika? Fragments of herself mixed with fragments of the bug? Essentially equivalent of late stage of Alzheimer disease.
I don't see any future for her relationship with Ayumi besides a drone or a soulless, expendable warrior.
I guess it would depend on whether the author states that there's a way to come back from that, or if it's basically a matter of being reborn. Which, if that's the case, then would she be "Reika" anymore, which defeats the purpose, agreed.
Maybe it'll be handled differently in the manga - instead of that specific turn of events, perhaps something like:
Reika starts losing, and her bug--which I would assume would be loyal to Towa and the Pieris Rapae to the point of fanaticism--fully hijacks her, body & mind, in an effort to prevent its old loyalties being overwritten.
Reika "disappears" under the will of the bug, but powers up a whole bunch and becomes effectively suicidal in her attempts to kill Ayumi--and has to be basically put down to get her to stop.
She gets beaten at last, and is "freed" from the bug's influence--but is near-death, and the only way to save her is to re-dye her, which saves her life but leaves her broken with the bug now...I guess "factory reset"?--thus beginning the process of rehabilitating her, which Ayumi could undertake personally with the intent of "healing" her, and not merely converting her over to the new brood.
Again, dunno if it's
good, but maybe that would set up a potential path to them reconnecting. And if it turns out the bug initially
made Reika go Full Zealot for Towa in the first place (abnormal larva? no clue if that's even at thing that can happen, so I won't go injecting lore wholecloth beyond just the guess), then Ayumi "resetting/overwriting" the larve by re-dying Reika would ostensibly allow for Reika to come back as her old self.
But I'd have to go back and re-read, because now I don't remember if Reika didn't always exhibit subtle hints that she didn't actually like Ayumi, and only went fully mask-off once she become a butterfly under Towa--and if that's actually the case, then any future of them would require Reika's personality be overwritten in general, which would feel ....well, "bad", to put it mildly.
In any case, I can theorycraft and unintentionally make up lore 'til the pillbugs come home, so I guess it's best to just wait and see and hope it makes sense, all said and done.