Nationalism wouldn't end with a USE. Why? Look at Ex-Yugoslavia. Bosnia & Herzegovina, Croatia, Slovenia, Serbia, Montenegro, Kosovo, Macedonia - the majority of the southern slav countries were one big state for many years dating back to the Kingdom of Yugoslavia after WWI, but in the end war still broke out. The Ustaša & Četnici both were fucking big & being in the Federativna Narodna Republika Jugoslavija & later Socijalistička Federativna Republika Jugoslavija didn't change shit & ultimately just made them bigger. The SFRJ went to shit after Tito died & war broke out which turned into the largest massacre/genocide in Europe since WW2.
The countries wanted to be independent & in the end we had 4 fucking wars that destroyed so, so many lives & we can still see & feel the end result it all had. Nationalism doesn't go away if you group together every fucking country - even if they have a similar language & similar cultures. Don't even get me started on how this all pretty much started due to different ethnic groups & religions & nationalists of fucking course not being okay with it. Before you could actually try to do something like this you'd need to get rid of the nationalism in a fuckton of countries unless you want wars to break out further down the line & I quite frankly don't believe that's possible.
I might be biased, because I had a lot of family suffer while Yugoslavia existed & when the wars broke out, but yeah. Knowing in-depth the whole history of Yugoslavia I don't see this working well nowadays. I'm a socialist & I don't necessarily think this is bad on paper, but I know too well how awful this can end. Maybe it could work with a few countries like some Western Europe ones, but as soon as you add southern &/or eastern slavs you can forget about it working longterm without problems. Also, personally, I think it's too culturally diverse & stripping all countries of their cultures & ignoring their histories just to have them be a number is awful & just adds even more fuel to the fire.