I don’t disagree with you either. But I think that when people give their phone number to a business they are often unaware of all of the nefarious ways it will be used, whether by ignorance or deception.
My point is just that replaceable usernames are perfect for random people you meet IRL or on the Internet because it’s minimal trouble to disconnect permanently when the time comes.
I love Signal and use it with my family which is the majority of my messaging. But I was surprised to find out the rest of my family on iPhone are missing features I have on Android. This doesn’t help bring the iPhone users.