Particularly, the same kind of threat to other accounts belonging to the same person exists with username/password combinations as with email/password combinations; after all, people reuse username/password combos as much as email/password combos.

So, the same general class of people who you endanger by not storing email/password securely are endangered if you stop storying email and just have username/password.

And a lot of that class will have emails that can be quickly guessed by appending one of "outlook.com", "gmail.com" or some other popular free-webmail provider to the username, because if they reuse usernames and passwords, its quite likely they do it on their mail site and that they have a webmail provider. So while what Euler has done clearly has a significant convenience impact, it has negligible security impact.