Unfortunately, the US has so many current problems it needs to work out that America First means America Alone until then. Unfortunately, I don't envision any of said problems being solved in any way.
From what I could tell, -most- America First proponents just got mad that the country was sending/spending money abroad while our own veterans/homeless/sick suffered. Some thought it reasonable to give up being the world police if it meant fixing issues at home. I think both are a fairly reasonable ask/thought. The problem of course is that there seems to be zero movement in fixing anything people were complaining about.
That said, it's a policy also historically used by racists and ultra nationalists, so it muddies the waters a lot. But I also have trouble believing the majority of the 67% of Americans who supported the idea fall into either of those camps.
When "then" comes, the world will have moved on.