You both seem to be assuming that competitive pressures from other governments is what causes current governments to be stable. However, that seems pretty unlikely to me. I doubt the US government would be significantly different, even if there was no ability for other governments to compete with the US at all (eg. no migration, no trade, no military). After all, how does the US government currently compete? Obviously with the military, but the US government isn’t becoming less corrupt to avoid being out competed with the military. Aside from that, migration seems to be the main way, and if anything the US government attempts to be less fit in that regard.
The forces that keep it stable are rather entirely internal. Similarly, a world government would be kept stable through the forces of politics—presumably some form of democracy.
You both seem to be assuming that competitive pressures from other governments is what causes current governments to be stable
Current, yes, but I was explicit that I don’t think this is a universal truth. As I wrote, “I think a well constructed world government could survive just fine without competitive pressure”.
I definitely think it’s true of your example though. If the US government was completely isolated, then the individual states would have a much-reduced incentive to participate in that government, and a much-increased incentive to defect and try and grab more of the pie for themselves. I suspect in the long term of that scenario, the US would dissolve into a collection of smaller competing states.
I think it’s misleading to say the competitive pressures themselves cause stability. It’s more that they provide the incentive to coordinate effectively, which is what causes stability.