Germany had its biggest boom when there was no Schengen agreement. Most of the German labor came from Turkey, not from the smaller and less developed EU countries. Its immigration policy was targetted and more selective even. Germany doesn't import much energy from less developed EU countries either: https://www.iea.org/countries/germany/energy-mix#where-does-... most of them come from EU-6 and UK which makes sense since those countries have technology and resources to produce extra capacity. It's the same deal with France with its former and current colonies. They truly do not need to be in a union with the less developed countries to get those benefits. Same for Canada, Canada doesn't need to enter a union with a less developed country to get lots of immigration.

I'm not saying that Germany (or other EU-6) doesn't need immigration. I'm an immigrant in Germany and I do support it for qualified and even non-qualified jobs. However, it is not a clear cut benefit to be in a union with emigrant source countries either.

The current setup of EU is a toxic relationship for both sides though, it is a benevolent colonization setup. Allowing smaller post-Soviet countries without significantly investing and improving their economies and industries and their political stability before ascension, ended up very badly for the other ones. EU-6 siphoned out all the labor, younger population and educated classes of post-Soviet countries, so now their populations are mostly old, resented people, the biggest businesses owned by EU-6 for only cheap labor. Those populations are really susceptible to authoritarian overtakes and the authoritarian governments like Hungary and Slovakia of today and Poland of past can block significant decisions with the veto right.

EU is very beneficial for smaller countries however at a significant cost for both sides in a bad way. It worked best when the candidate / new member nation was already a significantly developed and industrialized part of Soviet Union like Baltics or Poland (for the most part, they are not 100% clear yet).