Weaker Europe means less money for business subsidies e.g. agriculture, less travel and tourism, less household income to spend on consumables, less trade.
And how exactly does a weaker Euro benefit those countries ?
Also weaker EU does not mean collapse of EU, so tourism would be the same, as would be trade.
I am not necessarily an advocate of this solution, however, just pointing out that the current status of EU is not the best possible world for each one of its 27 members States.
Other say that that in the end it's always a zero sum game, as you do.
However, in the latter case, given that participation to the EU is on a voluntary basis, more and more countries may decide to exit.