You are making some assumptions. What you are stating as downside risks, sound like benefits to many.Yes, but I don’t think that USA GDP per capita being the highest in 1900 means it will be the highest in 2000. USA in 1900 imported massive amount of foreign expertise through immigration like it do today gaining a lot without having to invest, but without the dollars as the world’s reserve currency and de facto captured market abroad, USA will be in a completely different circumstance by TTL 2000. Without OTL Industrial-Military complex California will be a far different state, no Silicon Valley for one thing, with USA being a debitor nation Wall Street will be a less powerful actor, with a BOP deficit USA will be more dependent on domestic manufacturing and will have to stay competitive to export and gain foreign currency. More manufacturing means stronger unions and labor laws. All in all I expect a slightly smaller population, mostly because USA will see less immigration In the late 20th century (but it mostly make up for it with more earlier).
Here’s also the really major difference; without the Great War and the restriction in immigration and fewer potential immigrants in the interbellum. The Great Migration will be massively weaken, the South will be blacker and the rest of USA whiter. USA will also care less about external pressure for civil right to black people without the Cold War, so we will likely see segregation lasting longer and its end being much uglier, we likely also see much more Black Separatism with several states being majority black. Of course segregation and the civil rights movement will be seen very much as a Southern issue.
UK on the other hand will get rid of a lot of the dross in the decolonization (colonies owned for strategic reasons which cost more to administrate than they gives in), end up with control of much of the world’s oil production, London as the world’s financial centrum, and having the pound as the world’s reserve currency. Of course I also suspect that the median Briton won’t be better off and it’s simply the elite being richer and bigger.
US per capita GDP more then doubled between 1870 and 1910. The US was protectionist at this time. Tariffs were a principal revenue source and much of US production was to satisfy domestic demand. A lot of US trade policy post WW2 has been the US supporting other countries for security benefits at the expense of economic interests.
I don't know if you get the immigration restriction in the 1920s with no World War. Can't really call what population would be now. No Great Depression is a bigger factor then anything else.