Case in point, Pyrenean Gascon spoken in the High Pyrenees of France. It is apparently a separate language, unintelligible even to the Gascon spoken on the plains. Gascon is a language within Occitan that is spoken in southwestern France near the Spanish border in a region called Gascony. Gascon is probably at least 3 separate languages in itself. Gascon is often said to be quite healthy, with up to 500,000 speakers.
However, these figures are very misleading as the language is in bad shape in France. In Spain, where a dialect called Aranese is recognized as an official language of Spain in the Aran Valley west of Andorra on the French border, the language is in much better shape as it is still spoken by children.
For instance, in the High Pyrenees, only 20 years ago, 40% of the population spoke Pyrenean Gascon. That sounds like a lot, until you look at population figures for the region. Nearly 40% of the population at that time was elderly, as demographics collapsed and young people moved away from the dying rural area to the cities. Only 20 years later, the % of the population speaking the language had dropped from 40% to 1%!
How did this happen? Nearly half the population was elderly, and so were 99% of the speakers of the language. By 2011, in a space of only 20 years, nearly all of that 40% of the population that spoke the language was dead. Once the overwhelming majority of your speakers are over age 65, your language is going to collapse in about 20 years. Think about it.
Amazing. Language speakers collapsed from 40% to 1% in only 20 years. But if you understand demographics, it makes complete sense.
*Note: Careful with the links. Some of them are in French. I can sort of meander my way through French, but you may not be able to.