Once upon a time, there was the Kingdom of Aragon and the Kingdom of Castilla. With the marriage of Fernán e Isabel, these two became what is now modern Spain.
Both benefitted, and both benefit now from this arrangement. But the left hates it.
Catalan, btw, was a dying rural language that was revived by urban intellectuals who probably had no idea that it was going to be used this way. They just thought it would be fun and enhance their Spanishness (I have Catalan friends from the catalnista generation and they have all told me this.)
Also, some of the best writers in the Spanish language have been Catalans (or Basques). So getting rid of Spanish and having to invent a modern Catalan language, is like the Irish dumping English. In English, they were supreme, but now they write in a minor language that nobody reads and few can translate and nobody cares about. Of course, like Cataluña, the Irish have a huge Muslim population, ready to take up the slack...
In the mid ‘70s I worked for a short time in Ireland. That was just at the start of the Gaelic language movement. Most all I talked to thought it was a stupid move.
Fast forward to Catalan. We just finished the brilliant Netflix series called “If I hadn’t Met You”. We hate dubbed movies, so we listened in Catalan.
I “get by” in Spanish and am awful in French. My wife is fluent in Spanish and “gets by” in French. We were both pretty much lost for most of the movie and impressed the hell out of ourselves whenever we could pick out a Spanish or French phrase.
Good - just what we need - another patois. We’re already bombarded with French Creole in Florida.