Benevolence doesn't make a global leader. Having your fate intertwined with that of others makes a leader. The USA has always had a desire for self-sufficiency, and an economic approach that views any gains made by other countries as a competitive loss.
It is not to say that the USA is a terrible country, or that it is 'evil' or anything so silly. It is simply that the USA does not want a global leadership role, appears uncomfortable with it thrust upon it, and approaches global treaties as something it should either accede to or reject rather than change.
The Paris accords, ought to have heavily influenced by the 'global leader' so the preeminent power would view the treaty as one that benefitted its world view. As it is, the US world view is absent from the Paris treaty---why is that? I'd say it's because the US abdicated its leadership position to the point where European interests and American interests no longer seem to align.
> The US isn't doing a great job, but if American power gets rolled back, are you ready for Russia to come rolling into Eastern Europe?
I think it is a mistake to completely discount the military and economic power of Germany, France, and the United Kingdom. One should remember that historically the German military alone was enough to overrun Russia to the point that the Eastern Front was the site of the most horrific battles and loss of life of the entire war.
> You ready for the Chinese to invade Taiwan? The US is the only thing preventing that.
This is a bit of a tangent. Taiwan is a small island off the coast of the mainland whose government considers itself "Real China" and the far more populous mainland to be the 'rebel forces'. The continued existence of Taiwan is because China finds it useful, not because of any US intervention. If war sparked out, the USA and Europe would likely do the safe and sensible thing by treating it as a civil war within a nuclear power.
All these tangents boil down to "What if World War 3 breaks out?".
I hope for all that is good that WW3 does not break out between nuclear powers, and if it does that the USA will finally stay out of a continental conflict that is nowhere near its borders.