I think some degree of erosion and globalisation is inevitable as long as there are countries bigger & stronger than other countries and they go out into the world (and countries being formed necessarily erodes some of the localisms, more so when they get centralised, as What The Country Is Like gets more standardised). I don't think there should be any specific 'level' that's the inevitable end point though, that all seems variable in who the bigger country is, and when, and what they did.
Like, it's not inevitable that the Welsh language is crushed as much as it was when England took over (though its likely), and it's very much not inevitable that the Welsh language has a huge revival. That required specific events. Now the country is officially bilingual and more & more young people are bilingual, it's very unlikely that the language will go away just because of globalisation or any other -ation. You need some other factor that takes out the language in Wales itself, and that would require straight-up oppression.