After all, there is no word for Hiraeth in English...
This is the main point to me. I'm English (with ancestry from a lot of different countries, none of which has bequeathed me a particular language due to assimilation). I live in England. I am never likely to absolutely need Welsh to communicate. But every language, no matter how few people speak it is a window on the world. Every language has its own ways of thinking and expressing and being and its own unique way of lighting up its own culture, and every language should be preserved as far as is possible, even if sadly some of the time that means preserving it in books because we don't have native speakers to illuminate their own little corner of human thought for us.
It makes me sad that some people think that a language is ever worth losing. It doesn't matter if it's a language that hardly anyone speaks. You're not just preserving the language itself, you're preserving a culture and way of thinking that might not be possible to preserve if the people concerned are all communicating in a different language.
What does hiraeth mean, btw?
I'm currently having an extension built by some extremely charming Romanian builders and enjoying building up a working knowledge of how to offer cups of coffee, biscuits, doughnuts and iced water in Romanian. I don't suppose I shall ever visit Romania unless I suddenly and unexpectedly gain a lot of free time and money, but it's FUN. And it means that my builders who are all a long way from home, haven't been over here for long and mainly rather bad at English are enjoying their time here and realising that not all English people are totally uninterested in any culture but their own. I see it as my little bit of reaching out to other cultures and ambassadoring for Britain, actually, poncetastic as that sounds.
I sincerely hope that were I ever to live in Wales I'd do my very best to pick up a decent smattering of Welsh. Even if you don't need it and even if you can only say hello and thank you and goodbye, it's still worthwhile.