The other important thing to realize is that Welsh was ‘formalized’ in 1620 when the Bible was translated into it[1]. This had the double effect of unifying the distinct regional dialects into ‘high Welsh’ (a distinction which exists to this day!) but also, crucially, providing a standard text which people could use to learn to read and write. It meant that Welsh was able to flourish into a full legal language, and not just used as a ‘home’ language.
This gave it a backbone which was not present in other ‘regional’ languages (such as its nearest relation, Cornish), which in turn have all but died out.
With respect to the original post, I note that the Bible was only translated into Occitan in the modern era [2], which would support the evidence of it still being a language only used at home, and not in formal situations.
[0] https://en.m.wikipedia.org/wiki/Dafydd_Iwan
[1] https://cy.m.wikipedia.org/wiki/Beibl
[2] https://en.m.wikipedia.org/wiki/Bible_translations_into_the_...
So, we slowly forgot our own language (the other parts of the country that were not dominated by the English to such a degree, actually look down on us a little bit, because we do not speak Welsh as our main dialect...).
Recently due to pushes from the government, more and more children are growing up and learning welsh, and it's now at the point that most jobs in wales are successfully requiring Welsh-speakers as part of their hiring.
It's completely possible to resurrect a dead language with native language speakers still in existence, it's faster than killing the language (It took 300 years to completely eradicate Welsh and only about 50 years of government efforts to resurrect it), it just requires the collective will to be there, and the wish to restore the culture (or some bastardized idealized version of it).
[1]: https://en.wikipedia.org/wiki/Welsh_Not