To be fair... Apple hasn't built an iMessage client for
iOS either...
iMessage is simply a protocol that runs
on top of Apple's included "Messages" app that provides
additional features in iPhone-to-iPhone conversations. (there's obviously more to it... but you get the drift)
You can just as easily send and receive messages with Android users as you can with iPhone users using this same singular "Messages" app. I do it all the time.
I can choose to turn
off iMessage in Settings and the "Messages" app becomes a simple SMS client that still has the same ability to reach both Android users and iPhone users.
The point is... iPhone users
are reachable by nearly any phone in the world. Already.
And that's why I find it funny that there's this idea that
"iMessage is an impenetrable wall that must be torn down!"
Is it though?
WhatsApp, Telegram, Signal, and other proprietary messaging platforms
are walled gardens. You cannot send a message to a user of those platforms without being a member yourself. It's the very essence of a walled garden.
Meanwhile... iMessage is simply a set of instructions that
enhance regular SMS conversations. And
anyone can send me a message if they know my phone number. Already.