Comment by hedgehog

Apple maintains iMessage compatibility with devices that are long out of support, if Beeper Mini is sufficiently similar to the client in for example iOS 12 then it makes an Apple decision to break Beeper fairly expensive. Even if they do the work to publish iMessage updates for the old iOS versions it just buys a little time before the new version gets reverse engineered, and that at the cost of poor user experience for the people with those devices in a form they will directly blame on Apple. Given all that I suspect he's right.