The loss of openness, the limiting of education and work opportunities, the rise of bad nationalism, the fracturing of European unity and community, the isolation of being stuck between the EU and the US, the inability to pop over to France for lunch or Germany for a football game, the loss of stature in an evolving world, the backwards steps to being more insular… I don’t know. All these things and more “may” come to pass.
I guess we’ll just have to wait and see.
I’d suggest “good” nationalism is having a bit of pride and care for where you live, a knowledge of your history, feeling comfortable in your ethnicity (as opposed to race) and welcoming other folk with similar aspirations and trying to improve life for everyone in your area… but that’s just my opinion.