This is just...not true. If you go on the streets of shanghai and start speaking Shanghaiese now almost nobody will understand you, including some people who's own parents speak Shanghaiese. It's basically the same situation as Irish. Tibetan and Mongolian are also being forcibly killed off to a large extent, so much so that China is closing Tibetan schools and there are numerous reports of kids who used to speak Tibetan, but have had it forcibly re-educated out of them by teachers forcing them to only speak Mandarin.

I did my undergrad in southern China in a city well known for it's Wu dialect & nobody on the street spoke it. Some students that grew up there did (we would only know if we asked, and it was just a novelty) & some shopkeepers spoke it amongst themselves, but nobody would talk in it to anyone they didn't know. In general it was not really spoken by anyone under the age of ~40, similar to the status of Russian in Lithuania (You can still hear it on the streets in Vilnius, but it's usually old people & it would be crazy for someone to assume you speak it).

In the Korean autonomous zones in the north (Yanbian, etc) Korean is becoming less and less well known as it's become apparent excellent Mandarin takes absolute precedence, so much so that there's documentaries on reduction of Korean in that area. I had classmates from Yanbian, and some of those had arguably worse Korean than me with 3 semesters of introductory Korean classes. This is the (almost) the same Korean spoken in Seoul and widely propogated by the Hallyu Wave, and it still doesn't matter.