> but this is actually not going to gain a lot of traction
You're forgetting the part where Apple is going to mandate that app developers must offer Sign In With Apple if the app offers any social logins like Facebook or Twitter, and that the Sign In With Apple option is strongly suggested to be the first one on the list.
Huh? Add a handful of lines of code in your iOS/Mac app to add another login button or implement a whole server based authentication scheme? Maintain and keep secure the server based authentication for the lifetime of your app? That is not at all likely given the relative cost in money and time between the two choices.
it will never be more popular than fb because they don't have the user base. And this only applies for new signups, generally there is no option to switch from fb sign in -> apple sign in.
You're forgetting the part where Apple is going to mandate that app developers must offer Sign In With Apple if the app offers any social logins like Facebook or Twitter, and that the Sign In With Apple option is strongly suggested to be the first one on the list.
It absolutely will gain traction.