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What causes the loss of a local variety (dialect or language) is not simply exposure to other varieties, but the loss of the identity associated with said variety. In other words, what led to the blending and death of those dialects wasn't the audio communication technology - it's economical, social, and ideological pressures, such as nationalism.

I'll exemplify this using rhoticity in England. If telephone, radio and TV led to blending and death of dialects, you'd expect rhoticity in England to increase, due to exposure to American media. It didn't - it's decreasing:

Source for the map: it's a collation of both maps in this article. The reason for the shift however becomes obvious when you look at identity matters: "you're a Brit, speak like a Brit".

The exact same reasoning applies to other languages, by the way. Caipira Portuguese features aren't being replaced with the ones from that weird Globo TV accent, but with the ones spoken in São Paulo city; sheísmo in Argentina seems to be spreading, regardless of media from other countries; Occitan was not killed in France by simply exposing kids to French, but by making them feel ashamed of speaking Occitan.

With that out of the way, it's hard to predict the future impact of machine text generation, be it through LLMs or better models. It's perfectly possible that this sort of tech helps the preservation of local varieties, as LLMs are kind of good at translation; for example, I've noticed that Gemini is able to parse Venetian, even if unable to answer in the language.