I think accents themselves will become a relic of the past, given the prevalence of mass media and the internet. Millennials will probably be the last broad generation to have any hint of an accent.
There's a general tendency of accents merging or becoming more similar but new accents develop out of that as well. I think you underestimate the effect everyday speech has on people. Take the Dutch for instance. They're famous for almost universally speaking English very well. The main reason for this is that in the Netherlands foreign language films, TV series etc. come with subtitles instead of a dubbed version. Still many still have a recognisably Dutch accent.
Then there's another phenomenon that in language history so far has only happened to English: There are more non-native speakers than native speakers. So we might very well end up with something like a universal, worldwide English accent. It might just not sound like any of the native English accents at all.
That is a wonderful point that I've never thought of before.
You can really envision a lingua franca that is English but not American English or British English but Global English.
Second and third generation English speakers who live in a non-English speaking country as a majority of English speakers - will they be like how English is used in India or something completely different?
Fascinating.
Many small dialects are disappearing, but others are becoming more divergent (out west). It's a mixed bag.
(Sometimes, I feel surprised when I meet someone else who was born in the city I live in. But if I listen to how someone speaks, no matter if they're fifteen or fifty, I can tell whether they're from here.)