If a government renders the government irrelevant they take away their own legitimation.

Sure in past that append a common term for that is "revolution" or "coup".

... and Catalan doesn't have a military position to enforce its influence.

The United States was an exception, but it was an exception borne of geographic distance and fighting against a parent already beset by its adjacent neighbors. Catalan will have no such military advantages if people try to settle this dispute via civil war.