Think it's like this: A huge amount of TikTok/YouTube stuff is produced by non-native speakers and the most popular stuff usually uses very simple lexis and syntax to appeal to the widest audience. Young people are being exposed to that & older people aren't at all, so there's a generational divide.

This still sounds like koineization more than anything else though...

General American has already lost a lot of features found in other dialects of English (past simple/present perfect distinction; adjective/adverb distinction; arguably the past perfect as a whole; mergers of several phonemes). They already find it hard to understand Anglophone foreigners.