A couple potential catastrophes I see still being possible in this scenario:
Gradual disempowerment. (But enforced peace might make this easier to solve, by reducing pressure on states to economically compete.)
AI-assisted propaganda / memetic warfare; e.g., in the centralized case, a [US/China-controlled] superintelligence tasked with culturally disrupting [China/the US] could do extremely bad things. (The night watchman could prevent this, but determining what constitutes illegitimate vs. legitimate influence seems more ambiguous, and maybe more risking lock-in if done slightly wrong, than other things here.)
A couple potential catastrophes I see still being possible in this scenario:
Gradual disempowerment. (But enforced peace might make this easier to solve, by reducing pressure on states to economically compete.)
AI-assisted propaganda / memetic warfare; e.g., in the centralized case, a [US/China-controlled] superintelligence tasked with culturally disrupting [China/the US] could do extremely bad things. (The night watchman could prevent this, but determining what constitutes illegitimate vs. legitimate influence seems more ambiguous, and maybe more risking lock-in if done slightly wrong, than other things here.)