Fair enough, but, you then have the problem of not being able to cater to people with feature phones. I think a combined email or SMS based options solve most problem cases.

I don't understand the hate for SMS, like "how are these people in tech?" -- most banks I know of work on SMS based OTP. SMS also gives you a sorr of a real identity to work with and physical location in most common cases (not the app provider, but investigative authorities if they get involved).