Consider the area that includes Western Washington and Western Oregon. As many of us know, most English-speakers who were raised in this area speak more or less the same variety of English. Contrast this with English speakers who were raised in the area that includes New York and Boston. This area was settled for longer than the Pacific Northwest, so we have more dialect differentiation.
I'm not asking whether modern telecommunications and travel would tend to homogenize the various varieties of English that are already well-differentiated.
I'm asking whether there's any evidence that modern telecommunications would slow down dialect differentiation in areas like the Pacific Northwest west of the Cascades where the varieties of English that people raised in this area speak don't yet differ significantly.