Perhaps I wasn't clear, but I couldn't care less what they use to message their dentist. I care that they use Signal to message me.
Installing an app is more than a 2 minute commitment. You also have to be OK with that app running on your device.
An average person will have pages of apps installed, but that would actually use only what's on the first page. So they have a choice of bumping something off their precious FP to fit my special app or just keep it where it lands after installation, on the back pages. And if it ends up there, out of rotation, it won't see much voluntary use.