The paradox here is that people are more familiar with the appearance of home-grown-style login systems and are more willing to follow through on those than the

The paradox here is that people are more familiar with the appearance of home-grown-style login systems and are more willing to follow through on those than the novel Persona flow, even though the security characteristics of Persona are stronger. It's a chicken and egg problem, and until someone really big takes the plunge and gets everyone comfortable with this style, anyone implementing it is going to be somewhat of a cost to signups.