> Phone numbers are becoming increasingly useless.

Not really, there's a ton of government services that require you to have a phone number (depending on where you live). I don't see any real suggestion for an alternative to having a phone number. If nothing else, to receiving notification. You can't really rely on iMessage, WhatsApp, Signal and similar services, you need one system that you're sure will cover 98% for all people. 3. parties can't even integrate into many of these services.

You could use email, but I don't really see how that's any better and many seniors will use SMS, but not email to any great extend.

SMS is still the only unified messaging service you can be sure that all your friends and family will have.