The notion that “Catalonia would fail on its own” is, frankly, ridiculous. There’s no reason why Catalonia can’t be independent and successful, same as other small nations, including other small nations in Europe. As currently envisioned – and everything about Catalonia is speculative for now – this “nation” would certainly be larger than Luxembourg, and probably about the same size as Belgium. Neither of those nations is at risk of being a failed state or economy.
That’s not to say that success is automatic, either. I know nothing of the Catalan economy or culture (except what I learned over four years of basic high school Spanish – which included some introduction to Spanish culture – over 45 years ago, and the bits and pieces picked up since then). But other small nations have existed for a long time and have thrived: Aside from those already mentioned, Singapore may be the most relevant, but Hong Kong counts, too. (Hong Kong was part of the British Empire, but as far as I know the British did not have Hong Kong as a welfare client. I has been productive on its own for centuries.)
The difference that it might make is in the EU itself when it applies for membership (if it does, considering the drag that the EU has proven itself on independent nations), or NATO or even the UN – to the extent that the UN has any relevance any more.
It would also be interesting to see how it would address the issue of trans-Mediterranean migration from North Africa, because even though it’s not as likely a landing place as some other Southern European nations (such as Italy and Greece, for example, or other parts of Spain), if it gains independence then it may be a target for that kind of movement while things are in flux.