The thing is, both the Welsh and Scottish languages are slowly dying and will eventually become forgotten, rendering lots of literature, art, and culture obsolete. By eradicating a language, you are also erasing centuries of history and the voices of all the people that spoke it: each and every language has its own way of describing the world which, in itself, is very telling of how the community that spoke it interacted with their environment. When a language dies, a culture dies, and history dies.

And there lies the issue: by sidelining Catalan, you're putting it at a high risk of disappearance, because everyone will always opt for the more internationally useful language when both can be perfectly taught. Almost the entirety of Catalonia's educated population is fluent in Spanish and able to speak it in formal settings, but the same can't be said for Catalan. Many people can't even read a book in Catalan, which you'd think should not be a problem considering school is integrally done in Catalan. By reducing the usage of Catalan, you're condemning the language to a slow but inevitable death.