Well, if (when?) uber/lyft enter the market, you probably already have their apps, so this is a non-issue.

For others, I think if you have competent product people, you'd limit registration to a phone number and offer apple/android pay. With iOS 12, for example, it would be so seamless since it can auto-fill confirmation codes. So all you'd have to do is put in your phone number and you're logged into a new account.