If a country has gotten to the point that you would be targeted just for having an app installed, just changing to usernames wouldn't be enough, as users could be easily detected through network logging (thanks AWS and Google for killing domain fronting [0]). Your usecase sounds like it needs an app like Briar.
It's pushed as a secure messaging client, and so when it doesn't meet the standards for that people will mention it so potential users have the information needed for a risk assessment.
Usernames: Signal needs to store your social graph (in a privacy preserving fashion) on a central server.
So, paradoxically, phone numbers were the easiest and most secure option to start with.
Actually, there must be methods for distributed routing. How does TOR finally map a hidden service to an IP address?
Maybe there can be a dynamic DNS service for user accounts on an anonymous messaging network?
I think officially it’s just (2) and (1) is just a side effect, but I think it’s secretly more about (1)
I don't see this sim lockdown being a problem unless you want to create a new account or for some reason your account has been logged out requiring 2FA. In my understanding, telegram also asks for SMS for 2FA on re-login. Though the 2FA code will be simultaneously sent to any other device you've logged the account in.
Have there been any cases where the people have spoken out against such regulations, and they have been reversed?