The problem is not the account, but the mandatory phone number verification

Yes, it is. At least from the perspective of normal users.

The reason for WhatsApp (or Telegram or iMessage) becoming as big as it is was the convenience (later the network effect, of course, too) of just entering your phone number and then it just works™. No server selection, no password to remember, totally hassle free—that is the argument I get to hear very often.

And honestly, I have no idea, how we could provide a similar conveniance that is fool proof and secure and private.