I hear your frustration. Switching from email to username is going to take some getting used to.

Here's my take on this change:

Switching from email to username isn't intended to make accounts more secure. Rather, the goal is to make delta accounts a less desirable target for hacking. Many people use the same password for their email account as for their Delta account (as well as many other websites). Hackers use this to target the "weakest" link because gaining access to someone's email account is incredibly valuable.

To use an analogy, suppose someone pickpockets your wallet with your ID and credit card in it. They now have your name, address, and your method of payment. This is referred to as a "complete set" because it's all the information that's required to commit fraud. Getting access to someone's email account is similar. Un-pairing emails from passwords makes Delta a less attractive target for would-be hackers.