"The US and most European countries"
More likely European nations are still mostly ethnocentric whereby the clear majority of the populations identify with a group that is historically part of that territory.
Which is fundamentally different from US, Canada, Singapore and even Central/South American nations which have yet another aspect.
"Ancient Rome and Ancient China both prospered economically under these conditions. "
Ancient Rome invaded, plundered, occupied and demanded ongoing massive tributes from it's territories. Even most of the Italian peninsula were technically slaves.
The basis of 'prosperity' in Roman territories mostly boils down to physical security, i.e. 'Pax Romana'
"Nationalism in all it's incarnations has historically been an attempt to reassert tribal balance in favor of some formerly dominant but now sub-dominant group and it rarely ends well for that group."
Maybe you should offer some examples here, because I can't think of a single one.
- Quebec nationalism resulted in a 'quiet revolution' and an officially bilingual federal state.
- Scottish nationalism resulted in devolution.
- Nazi/German nationalism in both World Wars wasn't exactly the reassertion of a 'formerly dominant state' - moreover, now that France is weakened and the UK is 'out' - Germany's dominance of the EU is basically unchallenged.
- Russian nationalism has been fairly strong since the modern concept of nationalism.
- Nationalist movements throughout the post WW2 era resulted in the ostensible freedom of many powers from former colonial rule: Egypt, Libya, Algeria. (That list is very long.)
FYI the modern concept of 'Nationalism' is merely a civic orientation of that which existed before (and still does) i.e. 'ethnicity'.
Sweden and Finland do not share totally arbitrary borders.
The border between Finland and Sweden is such that the relative ethnic populations of Swedes and Finns are contiguous, i.e. Sweden is where the Swedish people live, Finland is where the Finns live - mostly.
The ethnocentric nature of the nation state is still obvious today by the fact that many ethnic Swedes for example live in Finland, and are demarcated as a separate group - not just people who 'speak a different language', bur rather, they are of a different ethnicity than Finns, although technically of the same legal nationality.
Though nationalism is very different in a place like the USS or Canada, it's still forms part of people's natural identity.
Family, tribe, ethnic group, 'nation' (be it ethnic or idealist) are part of who we are; to ignore this would be to ignore the very nature of who we are.
That said, some things, like Scientific knowledge tend to be humanist, i.e. transcending ethnicities, and of course we often need to be reminded of this.