The mom-test is a good one, I'll have to think more about it. The truth is the advantages and disadvantages of various authentication systems are subtle, and hard for a lot of technical people to understand, much less care about.
Apple is a commercial corporation, and one of the biggest (by market cap) companies in the world. That gives me confidence that they'll be around for a long time, have sufficient resources to invest in security and reliability, and they have a well-established reputation for a focus on security. They do other things I don't like[1], but I think this is one area where they're setting really good precedent.
In addition, it's going to be difficult getting any sites (outside of maybe the crypto/grey-market) to adopt an auth system that doesn't let them contact their users. This is also I think a big failing of IndieAuth.