The E2E promise was that with Google's backing, suddenly millions of people would/could encrypt and PGP would take a big step forward, making some forced upgrades in the rest of the community worthwhile. That is seeming less likely to happen now.
HN submission: https://news.ycombinator.com/item?id=13728327
(Genuinely hoping someone will find the good news I'm missing here)
Note I said research: I do not consider the current crop of secure messaging protocols to be viable alternatives to e-mail, as they all suffer from extreme centralization. Allowing unicorns to own and monopolize global infrastructure may appeal to silicon valley insiders, but it's not something the rest of us should be promoting.