There are an estimated 900 million iPhone users. If about 1/9th of the world is using those phones. Twitter has fewer users and remains pretty relevant.
Being installed by default doesn't make them a user
Just because the app is installed doesn't mean people actually use it as their main chat app.
All my EU contacts that have an iPhone use iMessage. Companies(100k+ employees) in the EU, that have an iPhone as a company phone, use iMessage.
Well that is certainly new to me. When I lived in the EU I knew precisely no one for which iMessage was the main mode of communication.
Usefulness to whom? The hundreds of millions of iOS users worldwide seem perfectly happy with it. I count myself in it.
Far from all the iOS users actually use iMessage as their main chat app.