I’m afraid history has proven you wrong on this more times than I can count. I’ll give you 3 famous examples:
NATIVE CANADIANS: During the 1600s, the Algonquin, the Iroquois and the Mohawk found their territory was being occupied by small groups of white-skinned aliens with mega-weapons that were light years ahead of their own, and carrying a deadly virus than nearly wiped the natives out. Instead of uniting against this threat, they chose to ally with different parts of it, so that they could use these mega-weapons to destroy each other. Most notably, the Algonquin fortress of Kitcisìpirinik, impregnible until the arrival of gunpowder, was attacked and almost destroyed by both the Mohawk and the Iroquois, and the Algonquin were eventually driven from their lands. Sound familiar?
THE HIGHLAND SCOTS: Various Highland Clans spent most of their history allying with various British forces so that they could get one over on their rivals. The most notorious (but not the only) instance was the infamous Glencoe Massacre of 1692, in which the Campbells working for the English massacred Clan MacDonald in an inverse version of the Red Wedding (the MacDonalds were hosting them at the time).
THE ROMAN INVASION OF BRITAIN: King Cogidubnus and Queen Cartimandua did very nicely out of siding with the Romans against their native rivals thank you very much. Cogidubnus got a lovely new palace at Fishbourne, and Cartimandua’s Brigantes gained control of northern Britain. And I’ll bet you didn’t know that the Iceni of Boudicca originally allied with the Romans to kick out the Catuvellauni and the Trinovantes from East Anglia. They took out preferential loans from Roman bankers, minted Roman-style coins and started building Roman-style villas. They only revolted after the Roman equivalent of the sub-prime scandal caused the likes of Seneca to call in their loans, and an over-zealous Roman tax collector sparked the revolt when, instead of simply collecting the half-share of his kingdom left to Rome by Boudicca’s husband Prasutagus and leaving her with the rest, he kicked her out on the streets and let his men rape her daughters. If that hadn’t happened, the Iceni would happily have kept using the Romans to beat the Trinovantes down instead of uniting with them against the Romans.
So I’m afraid history goes against you time and again. Look at Syria today - even in the face of ISIS, the Assad regime and Turkey would rather kill the Azeris and the Kurds than completely wipe out the existential threat looming in the east - and use the advanced tech of the Russians and Americans to do it.
In fact,the only thing the game appears to have got wrong is that the factions aren’t allying with the Pandas against each other. It sucks, but it’s human nature.
That doesn’t alter the fact, however, that it would be much more fun for most players to have a middle way where they could try to keep the peace. Personally, I’d like to see JG & the Devs include this as one of the 5 DLC - I’d call it ‘Rise of the Peacemakers’. When I have time, I’m going to propose it on this forum. If they don’t do it, it would be lovely if some modder took it up.