1) Can? I think so. Should? I’m not too sure. Obviously, there are people who don’t have those “social” accounts or don’t want to use them even if they’ve somehow signed up for one. The big question though is not their existence, but their numbers in the potential user base.

By removing adoption friction for one group, you are potentially adding a friction point for another. You’ll have to be the judge on whether it’s worth it.

How about email-only passwordless authentication? Seems like a viable alternative to the username/password. And yeah, keep the social logins as well on top of that.

2) Yeah, that necessity depends on your product.

In what cases or features is authentication needed in a food-ordering app? Storing user addresses and past orders for quicker reordering later? Implementing a loyalty point system?

If in your authentication-less app, you can provide similar features and benefits provided by an authenticated app without compromising much, go for it. Or just cut features and make simplicity your selling point.