Example: Most English accents today don't pronounce an R if a vowel doesn't follow it ("non rhotic"). This is also true in accents of New York, New England, and parts of the US southeast.
However, a few hundred years ago English accents were all rhotic, like the majority of the US. I had heard somewhere that the non-rhotacism was actually adopted in the northeast and affluent south after it became popular in England and also after colonization by people who would have pronounced all the Rs. That is to say some well to do US accents kept up with changes occurring in places like London.
Outside of some parts of Boston we pick up those Rs our friends drop and put them to use.
I grew up with a western mass accent, which is a sort of lighter version of bahston with less non-rhoticity. I can turn it off, but lately choose not to, also for social class reasons.
Distinctive communities get broken up now and disrupted by media.
Not so. Maybe this is true in many cases, but I personally know of at least one outlier: a relative who grew up in West Virginia, lost her accent to work in radio and news, and when older re-married to a man from Texas with a heavy accent and developed one herself.
It is also the case that many people slip back into accents as they age.