"It was never as difficult as portrayed" - we are talking about UX here. Stating that something is not as difficult is wrong. The UX is bad. When you go to a new country for the first time, you need to search online or ask around how and where to buy sim card. Yes, you can do it in a lot of countries in almost any grocery store. I'm not saying it's easy and it's close to impossible to have something like this implemented world wide. I'm proposing what I think would be ideal solution.

Login/password is the current de-facto standard. Until something better, widely accepted comes along, I don't see why my phone number should be safer than my facebook account or my bank account (it's not username/password, but the security of cards that we use for payment is pretty bad)