1. Limiting identifiers to email addresses isn't that great a solution. Email is less popular amongst my generation (~20 y/o). It's the service backing almost every identity now, but I already have way too many email addresses which I have to check. What if I want to set up an anon identity? 2. Why, as a person on the street, would I trust pgp.mit.edu more than I trust Keybase THEN (Twitter AND a personal webpage AND whatever other services end up being supported) - it does ultimately depend on how much you trust Keybase, but not obviously so.
Looking back on it in a year or two, Keybase may end up a classic example of "worse is better", because it's easier to grasp, as you said in your post.