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.