"Sure, every website can implement this flow, and users could could go to every website they've ever logged into and update their email address... assuming it all works properly even though they might not have access to the old email address anymore."

And how is this different than the current situation? Nearly all web sites require an email address. With BrowserID, you at some point confirmed ownership of that email address, so you could continue to use it to login, then change when you're ready.

"At the very best this technology offers considerably less value to websites and more hassle to users than Facebook or Google. And it's about 5 years too late."

Tell that to users who a) don't have Facebook accounts or b) don't want to use Google or Facebook with their identity. Far more people have email addresses than Facebook or Google accounts.