[persona]

I'd argue that this makes more sense in the real world than in a diagram, after being burnt in multiple real life experiences :)

The assumption both you and AnyList are making is that an email is "THE obvious choice". From a user experience perspective perhaps this "global sharing identifier" should be defined by them.

You'll notice that different generations have different online behaviors. For some, email is their main id. For others, it's their phone number (they don't know most of their friends' e-mail, but know their phone). For others, it's either online handles or nothing at all - think about the device set up for grandma with her daily To Do list.

Of course, having this approach would add some upfront dev work to them but allow them to navigate this much easier later on. And for anyone starting to develop their new app/site/product thinking about this early on can reduce a lot of future headaches.

[JAlexoid]

That's a very idealistic opinion.

People don't care about usernames and other crap. They want an easy option - enter email, communicate over email and be found using email. This isn't their banking app or anything that important.

[filleduchaos]

Nobody "wants to use their email". They just want to start using whatever app or service they're trying to connect to as quickly as possible - why exactly do you think "Sign in with X" became so popular in the first place?