Paul Graham said that email was horrible, and it shouldn't be hard to come up with something better. But he didn't actually put forward a concrete proposal.
So: What would be better? Given that the world currently runs on email, what would be enough better that it would be worth it to switch? I don't know. I don't think Paul Graham knew then, or knows now. And I think that the reason people haven't switched is that, so far, there isn't an alternative that's enough better. And the reason there isn't one is because, contrary to Paul Graham's expectation, creating something better is actually pretty hard.
So: What would be better? Given that the world currently runs on email, what would be enough better that it would be worth it to switch? I don't know. I don't think Paul Graham knew then, or knows now. And I think that the reason people haven't switched is that, so far, there isn't an alternative that's enough better. And the reason there isn't one is because, contrary to Paul Graham's expectation, creating something better is actually pretty hard.