[2019] european Union?

Yes, Sweden can have it's own DD line (maybe with a split), but it's still be just one line, instead of other big nations having most classes fleshed out. or even several lines for one class. Similar situation with Netherlands, Spain, and, to some degree, Poland (with 3 variations of Grom-class). But even if those nations would've gotten their own trees - there were nations with less ships, and therefore European tree would've existed anyway.
And even more, getting all those nations into one tree is better for players - interface wouldn't get cluttered, commanders are recruited with the uniform relevant to the ship they begin their "career" on, you can freely swap them, and put on flag of your preferences.