Why not replaceable? iMessage seems like a perfect replacement.
Edit: for Android devices, Signal is just as easy to use as WhatsApp.
Only if 100% of your contacts use iPhones.
Not replaceable in a sense that there are some people that are only reachable over WhatsApp and Messenger.
Signal is far more annoying to use and is less consistent about notifications. Accordingly, most people are hard to motivate into switching (or installing yet another messaging app).
Messenger is so much better than the default SMS app on my Android device that I can't get rid of it. I don't know if it's just US Cellular crapware, Samsung crapware, or Android core crapware, but the system default is garbage.