I recently got locked out of my old bank account because I didn't use the SIM from my country of origin for 5 months. My mobile operator disabled my ~15yo phone number and I can't recover it without flying there and visiting their office with my ID.

That's why I also feel uneasy towards all these number-based messengers. They're targeted at users who rarely leave their countries.

Or at least get a mail address or similar from everyone whom you're talking to on a number-based messenger, so that you can recover from a lost messenger account.

(Doesn't help, though, if you lose the phone number and your mail account at the same time.)

exactly. if you pay for a service and google or facebook are your authentication, you are locked out of the third party services you pay for.