If I am told in January that I can transfer money to you using just your phone number, surely that is good?
If we have to opt-in to the service, a lot of people won't opt in due to inertia.
And opting in would be a field day for scammers. You would get all sorts of texts and emails inviting you to opt in and next thing your bank account would be cleared.
What is the disadvantage of this payment scheme? Is it vulnerable to more fraud than the ordinary bank payment system? I have the AIB app on my phone. A fraudster would have to be able to access it and then transfer money to his phone number. But if he can access the AIB app on my phone, then he can transfer it to another bank account anyway. So it doesn't look more vulnerable.
And if there is a disadvantage, then you can opt out.