The US has a presidential position that is too strong (the US-type presidential system is often called one of the worst US exports) but there are long established institutions and mechanisms that keep the ball going. Trump(ism) found gaps and there are enough holes to make a mess out of things like transfer of power, but it's what keeping America from falling apart.

Remember JFK, transfer of power after Nixon, even the Bush/Gore debacle. The whole machinery worked in the end, however flawed.

Totalitarian regimes like China, where you dismantle your opposition, including state institutions, make them very vulnerable. If Xi or Putin disappear tomorrow, there would be utter chaos.