You don't need to be specific to the Created when talking about the mechanism to progress the setting and the narrative beyond the post-war doldrums that the franchise has been languishing within for a decade now. The mechanism is generally just some kind of out-of-context issue that no one in the current setting is equipped to deal with singlehandedly in their current state. It could have been an external power to the Orion Arm, but I'm not keen on that happening in Halo just yet. I think the time for that can certainly come, and it can come with its own benefits in shedding light on the vastness of space and the depth of time this galaxy has seen, and our perilous position inside it when one ponders what else could be out there besides the Covenant. However right now I'd prefer that the focus be more on establishing the current players a bit more.
The Created are another solution, but one which I have reservations about in the way that it will likely lead to AI in the setting being thrown under the bus. It could be that a new "Covenant" emerges, but you can bet that "substrate prejudice" (to steal a Banks' phrase) will lead to the suppression of AI technology, the lobotomization and erasure of AI beings and a disregard for thinking beings that are not organically based. However fine, we'll ignore the wealth of science-fiction work that explores this concept and classify AI as just being software. I guess things will be fine then.
What won't be fine is the lasting damage this will do to the image of humans in the eyes of the Orion Arm and the vindication of Old Covenant doctrine regarding the dangers of sapient AI. It would be a reasonable bet that the issue of the Created being a human problem will be brought up in future, and to be fair to the future writers who may pen this idea it's not unreasonable to consider. The prejudice that humans will face for being viewed as having brought the Created upon the Orion Arm won't be something that could be overlooked, and requires the splitting of far fewer hairs than quite a lot of the drama that has been introduced to this franchise since 2011. That likelihood may torpedo hopes for a new era of political rejuvenation in the setting that could bring the notions of cooperation and of not looking into the rear-view mirror.
I don't see why the defeat of the Created, or of any outside threat to be honest, would require or result in the radical kinds of cultural shifts that the Orion Arm needs in order to become stable and merely a place where one can live in peace to say nothing of thrive in. I don't really believe that outside threats can solve longstanding structural and cultural problems within a society, unless they are so great and shocking - I think they can only come from within. So, in place of the Created or another alien menace I would have liked to have seen an out-of-context issue come as a result of the collapse of the Covenant such as another Unggoy rebellion, or the collapse of the region's economy and trade resulting in severe resource shortages, or the unleashing of the Old Covenant's weapons upon former Covenant worlds as the civil and turf wars get completely out of hand, which all could force a post-WWII styled sense of reflection that "If we keep doing these wars with these kinds of weapons, and tomorrow's weapons, we'll eventually kill ourselves." People then change because they realise that if they don't then their beliefs or the perpetuation of those beliefs could get them or their loved ones killed one day. I don't see how the defeat of the Created will clearly dissuade Sangheili warlords about throwing Unggoy out the airlock in quite the same way as a massive and uncontrollable Unggoy uprising (Violent or civil) taking the entire Orion Arm hostage would. Nor is rampant and often genocidal racism, xenophobia and racial-nationalism going to go away just because some AI briefly took over but have now been erased or whatever; not in the same way that a war resulting from those beliefs seizing control of fleets capable of turning worlds into molten balls of magma and murdering billions could.
I guess I just don't see how the Created present an unbearable cost or potential cost to those who hold all the unsavoury beliefs rife within the current setting that prevent anything or anyone from being able to move on, in a way that makes those beliefs particularly problematic themselves. I guess you could say that holding these beliefs would prevent people from being able to unify to bring down the Created, but remember Kilo-5 killed that entire theme with its "The enemy of my enemy is not my friend but just a future competitor" bullshit. So they can all easily work together to bring down their common enemy then go right back to killing each other, destabilising each other, enslaving each other, ethnically cleansing each other, etc forever.
I don't know what the heads at 343i want with the story, but I see little indication that they intend to steer the setting down this path of a new Covenant or whatever, so I don't really see the Created as being utilised to achieve that. What they are being utilised for is something that I can't even guess at now.
With the Sangheili-Human alliance brought on by the Created, it seems to me that this story arc will end with the Orion Arm united against Cortana, ultimately defeating her and forming a new "Covenant" with the inclusion of Humanity serving as arbitrators in this new alliance.
I don't see why anyone would accept or listen to Arbitrators who are exclusively only allowed to be human. Even assuming good faith relations you would be asking people to just accept the moral authority of a random species for essentially no reason, a race that demonstrates no significantly outstanding moral or intellectual characteristics. You would be expecting them to trust that human arbiters would rule against themselves if that were to be the most reasonable and fair outcome, which is something that humans are no better known for than anyone else. You would be asking them to trust that human beings understand them, their culture and their alien mindsets better than an arbiter from their own kind could. You would essentially be asking them to submit to an authority within which they have no representation. You would be fixing the politics of the Orion Arm in a specific way that may lead to xenophobia further down the line, especially when these human arbiters make decisions that some may not agree with. Nothing in this is realistic, or desirable.
It's not just the Mantle itself or its past implementations that are wrong. It's also the way in which it comes served with some form of racial supremacy.