Releasing the Aether released the primals. The primal, or eikon, interference is the thing that ACTUALLY turned away the main Garlean invasion force (one popped up basically immediately after the conquering of Ala Mhigo and Garlemald went 'nope, fuck this, we out') and led to a division in Garlemald about whether Eorzea was worth the trouble, leaving only Nael (who wanted at Allag) and Gaius (who wants to save the world from the eikons) and, if memory serves, the new emperor, as the only members of the Garlean politick that actually think Eorzea is worth the trouble with those THINGS stomping about (and, honestly, with Ultima weapon broken, they're probably right to think Eorzea isn't).
The primals are also the only things really capable of resisting a concentrated Garlean assault. Or were at least. With Ironworks churning out magitek gear now, maybe that won't continue to be the case, but Eorzea (like everywhere else) has no real defense against concentrated Garlean assault given the massive differences in technology level and man power.
In other words, him smashing the seal is the reason this game takes place in Eorzea and not 'a prefecture of Garlemald'.
