I'm not necessarily disagreeing with you on it being bad storytelling, although I still think the Insurrectionists have very little to offer the Banished strategically. Credit for the quote citation for the Banished being at least not principally opposed to Human alliance, though, I don't recall ever having seen that and it is interesting.
The biggest reason I don't think we'll ever fight Human enemies in a mainline Halo game really boils down to the fact that I think MS is trying to keep Halo as a largely "humans good, aliens bad," paradigm. I think it's pretty debatable whether the postwar era is or isn't a good narrative evolution to the original H/C War (let alone better than it outright), but I do agree that rehashing an H/C War that's really just reframed as the H/B(anished) War is a bit of a cheap option, certainly from the perspective of narrative complexity.
Again, though, I think MS is less interested in narrative complexity and honoring a progressive postwar arc in the Lore that's been ongoing since H4 and more interested in trying to recapture the success of the original Halo games set in the H/C War (only this time, more marketable to the whole family). All the "Spiritual Reboot," stuff feels like it's hinting at that direction to me, along with all the stuff detailed in my first post. Some elements are almost sure to survive, like the UNSC's alliance with Arbiter's faction, but I do think a lot of the complexities between all of the disparate groups are deliberately going to be "reset," to a more easily digestible (for less lore-invested audiences) situation. Whether or not this is a good move isn't what I'm arguing, I'm just saying that I think it's what Microsoft is pushing for.
**Edit- It's not lost on me that the majority of my argument banks on the metanarrative side of things rather than specific Lore-based reasoning, and that may be a bit out of scope for the intended conversation since this is in the Lore topic, but to me it's a situation where the metanarrative and narrative are inseparable; the Lore won't be allowed to pursue Human vs Human hostilities beyond a pretty limited point if the people making the high level creative and marketing decisions won't allow it to branch into the main games.