If an "unsanctioned" client can compromise iMessage security, then there was no actual security other than obscurity.
Imagine there is a theme park that has normal ticket booths and some requirements there to get in. Then there comes a Beeper that finds a hole in the fence on the perimeter and sets up their ticket booths there. It's in theme park's best interest to close that hole and cut off the revenue stream of somebody pigging back on their theme park.
Okay, I've stretched the metaphor out enough.
A Lamborghini Urus costs $230k so I guess it's morally acceptable to break into a dealership and steal it.