While UN is not optimal and needs a revamp, it is an organization that has almost all countries of the world as members. There is no guarantee that a new organization is going to be any better. Once US leaves UN, why would any other country believe in US to build a better organization, especially seeing that the existing administration in US itself is chaotic.
^ This is the primary meta argument against Trump foreign policy.
If you screw your counterparty over in every negotiation, you erode trust and end up without allies.
That’s fine in a business setting, if you’re self-capitalized (although Trump famously ran into issues after burning bridges with most banks), because you can do without them.
It works less effectively in a forum of sovereign nations, where you’re going to need to deal with CountryX tomorrow and ten years from now.
The US is ceding the soft power and web of alliances that are the basis of its economic and hard power.
The US, without allies, loses to China strictly on the basis of population.
“America first” is “America alone” with an orange spray tan.
