I don’t think everyone understands the intricacies of minority languages co-existing with dominant languages. Especially when the dominant one has gone through periods of being imposed by the government as the only accepted language nationwide.
I come from Spain where there is a similar situation to the UK. Spanish is the main language but there are 3 co-official languages as well: Galician, Basque and Catalan/Valencian.
From 1939 until the 70’s there was a fascist dictatorship which meant anything not purely Spanish was ‘bad’, and forbidden. That’s a whole generation of people banned from speaking the co-official languages - which for many was their first native language as well.
Children didn’t learn it at school. People couldn’t speak it at work, at church, at a birthday party - for fear of being reported or arrested for having gone against the law.
The languages survived because many families kept speaking it at home, especially in rural towns and villages. When the democracy was established, there was a big push for these languages to be revived again. However many of my friends have only spoken Valencian with their grandparents, as their parents never became fully fluent in anything other than Spanish.
Yet even today you get Spanish ‘superiority’ from people who come to holiday on the coast, and get annoyed when the staff in shops speak Valencian as the default language. ‘It’s a Spanish shop, they should speak Spanish here’. (Staff will of course swap to Spanish when asked because it would be illegal not to).
They get annoyed when you need to achieve fluent Valencian to get a job in a school, a hospital, the post office, the police, or in the civil service. They get annoyed when they enrol their children in a local school and the newsletters are in Valencian, or the kids need to sit exams in it. They’re Spanish - they deserve an exclusively Spanish service on Spanish soil 
You go to say, Madrid with your friends - whom you’ve never spoken Spanish with in your entire life - and people tell you to stop speaking Valencian because you’re not in Valencia anymore. Sure, because crossing the county border makes me want to speak a different language to my friends or family.
Just because I’m fully fluent in both languages doesn’t mean I shall ditch the minority language when in a Spanish-only speaking area, so I’m not ‘rude’ to people there.
That’s how minority languages are lost, that’s how you oppress speakers of that minority language. By telling them their language is inferior and would you please mind switching to English when not in Wales, because it’s rude not to.
We need to fight to preserve minority languages because a huge chunk of our cultural identity comes from them. Art, literature, so much history and tradition will be lost if minority languages aren’t seen as NORMAL everywhere in the country. It should be NORMAL to hear Welsh, and Gaelic, and Irish, anywhere in the UK. People shouldn’t feel ashamed or judged or feel like they’ll be ‘othered’ if they don’t speak English in the UK. The UK isn’t just England. It’s this diverse mix of cultures and languages that everybody benefits from and plays a part in, regardless of how ‘Englishly-superior’ they feel.