You need some sort of verification that the person is a person. Phone number puts a layer between you and the service you are trying to use - the provider of the number. The provider holds your identity but only passes on a phone number.
It’s definitely not ideal, but not bad
WhatsApp, telegram really covers 99% of my contacts, private or work. My parents use sms (60)
Signal is not very wide spread, mostly used for texts you want to protect for whatever reason.
I would consider it very odd if somebody writes on iMessage , iMessage was shit for groups when WhatsApp did it better.