I actually experienced firsthand the last part of a cultural suppression of regional and/or city dialects in my native Flanders. Much has been said of Belgians push to establish standard French as the country's universal language, even at the detriment of the Dutch-speaking Population of Flanders and how eventually it misfired. However past WWII there was another wave of pan-Dutchism that aimed to unite all of Flanders (with all of the Netherlands) under a standard Dutch language.
The main force was a standardization of everything Flemish: national grocery supermarket chains replaced the local mom-and-pop stores. Local newspapers consolidated into just a handful of Brussels-based groups, local breweries one after the other were bought up by what is now ABB-Inbev and of course there was only one Flemish radio and TV-network (which was the Flemish subsidiarity of the state-run national Belgian one). Along this came a push to use one standard Dutch Language. THE standard Dutch language already in use in the Netherlands. After all, if Belgium and the Netherlands could only maintain their European presence by grouping up in the BENELUX Union, Flemish for sure must group up with Staaten-Bible Dutch to maintain a cultural significance.
And so it went for a good 25, 30, 35... Even almost 40 years: all major outlets used the standard Dutch, it was thought in schools as the only 'right' way of writing and therefore speaking and increasingly the regional dialects were seen as un-modern: a throwback to the times before supermarkets, plastics, cars or even indoor plumbing. Eventually, given 50 more years, like it had in France and Germany, regional dialects would simply go the way of the milkmen and donkey-carts. A victim of modern times no one really lamented.
Instead, we had the sixties and another aspect of the 1968 counter-culture was a rejection of the excesses of 'modernism' in favor of a more harmonious way of life. Harmonious with nature as well as harmonious with one's history and culture and eventually language. As with everything sixties, it took a slow change as the rebellious generation matured, moved into positions of power and although rejecting half of their previous radical ideas, kept adhering to the other half. But all through the seventies - which was my childhood- it kept growing and by 1980 environmentalism had become a thing, grocery chains reevaluated the idea of suburbian mega-shopping centers in favor of smaller inner-city stores and as far as the language was concerned, recording studios now actively signed singer-songwriters that brought their (literal) folk-songs in their local city dialects. Finally in 1985, when I was 18, the ministry of education dropped it's focus on teaching Netherlands Dutch as the only permittable language in favor of just teaching a common writing language. A development I personally would not even have noticed if not for the fact that at the same time as part of language theory and evolution, my Dutch class was given a two-month course on regional dialects and language research.
So looking back on my own experiences, I would say that suppression of regional language and dialects are cyclical thing: a constant pull between the forces of regionalism an uniformity. A pull between a strong centralized state and a federation of equal, but independent communities. A pull between the ideas of national duty and individual citizens right, between international superstars and local celebrities. Looking back at almost 200 years of Belgian history, it has always been there: first there was the nationalist culture celebrating shared Brussels greatness versus a bourgeoning romanticism celebrating local folklore, history and with it language Then an industrialism wanting to make language as uniform as standard railway gauges versus a rising working-folk movement -socialist as well as Catholic- going up against factory-owners that literally did not understand their worker's complaining. Then after WWI and WWII it was a movement of uniform national rebuilding -one style, one language- versus the 'voices' of the communities that bore most of the suffering and finally postwar, the forces were the 1950's international modernism going up against the 'sixties' emerging 'green' culture.
Currently the regionalists are on the upper hand. We see this in the decline of national megaproject versus the rise of citizens justice movements, declining megamall stores versus rising craft breweries, declining legacy TV channels versus YouTube and yes, more literature and music in local dialects. However not so long ago those shopping malls with their Sears and Macy's stores, those Walter Cronkites, highway city loops and World Trade Centers were seen as the sure way of telling times are good and will be getting only better. Who cares about dead rivers when you got city parks. And likewise who cares about a language no one speaks on TV. And I have the feeling that before too long these times will have a comeback and reginal languages once again will be silenced, if not suppressed. Some of them will even disappear, or just survive in some old sayings, rimes or concept words. But the other languages will find some way to change, adapt and survive,.. until the next cycle comes along.