Comment by VLM

Two related issues not mentioned yet:

Its really slow. Too slow to use 2FA or in some cases, verify email addresses or recover passwords.

Most people can't handle a notification on their watch every minute, or several spam every five minutes, so "large numbers of people" are shutting off notifications on their phones. And human nature being what it is, they're not going to be turned back on again. So the era of getting a notification when you get an email is coming to a close. "Important Immediate Attention Stuff" moved to text messages a long time ago anyway, at least for me. The list of technologies you can no longer reach me on, always increases over time...