Personally I would pay for this because it solves a ton of logistic problems if done the right way, can improve personal data protection for an ordinary person and it can save a decent amount of time and mental overhead. Time and mental capacity are precious for almost everybody, I believe. So I imagine an app that charges a fixed fee or a subscription (with backup restore via multisig for example)

Replacement of authentication in all consumer apps is probably a huge challenge. Unless there is a way to recreate a new identity ecosystem that will overgrow MSFT/APPL/GOOG.