[bwooce]

I struggle with this. I acknowledge all your points, but it doesn't cover the usecases of: 1. Changing ISP and getting a new email address. This is really common. 2. Having multiple addresses (work/home, etc). Also see #1

This breaks password resets and creates a "I want to change my credentials" flow that doesn't exist with usernames. It is especially complicated as emails to the old address won't work/are not accessible.

Most companies want to keep track customers over their lifetime and not have them create a new account when they change ISP/job.

If you want to see an example of this not working at all well, see Apple IDs. The pain surrounding them, purchases, @me.com, @mac.com, changing countries and the attached purchases is inspirational in its depth and breadth.