> It's very simple.

Is it? I doubt it. There isn't a large country on earth where the salaries don't differ across regions.

It's probably the most complex and impossible to solve take, it's not even true in the US or any other place in the world.

It doesn't even work in a single country for the simple reason that governments have very different ideas how to redistribute taxes. If one country can't do it well, how could the EU?