Part of the issue is that Spain & periphery tend to have fewer 'large industrial conglomerates' that enable profitable things to be done at scale (i.e. making cars etc.). % people employed at companies with >1000 people is much bigger in Northern Europe - which makes it more difficult to compete in the periphery.
But really, this is what can be expected with a true Union in Europe, and it's why Germany is so keen on it, and of course the Lib Dem (i.e. business types) are super keen on it in the UK, despite populist upset over Brexit.
The short run advantages of common market and supposed common risk of the Euro (which never happened), the downside is that skilled people are flooding out of the mediocre areas into the competitive areas.
Portugal is turning into a nation-sized retirement home, sadly.
In an odd way, this might actually be a 'net good thing' economically, because Germany might be better at putting people to productive use in industry ... which is why the Lib Dem types like it (i.e. they see the economics) ... but really this speaks to an existential challenge to the very nature of European nations and peoples.