You're setting a standard that can't be met by anyone. Any "concrete evidence" would have to provide concrete evidence of a hypothetical. But it isn't that bizarre of an idea that Germany could be better off if it wasn't tied to much weaker economies through a shared currency. Of course the problem here is that the situation is so large you can find not-quite-concrete evidence for almost any proposition.

Agreed, there is no "concrete evidence" and no way of supporting any of the sides. But let's just imagine for a while, what if it would be the exact same opposite?

Who helps richest countries go richer? How can a country grow if there is no decrease somewhere else? Maybe a greater profit if you don't need to pay for a different currency?

Edit: With help I mean get cheap labour, cheap goods, cheap rent and even being able to sell in that country's market and export abroad.