Comment by isodev
I'm not sure Apple can be a good steward of such an open, plug-in based solution - they would always put in some restrictions to make the process very complicated and not accessible to platforms and developers.
At the same time, making it possible to choose WhatsApp for the default messaging app has been a great relief for those not locked into Messages.
Well the thing is: if they were particularly obnoxious about their implementation, they could be replaced. I’m looking forward to a multi-protocol messaging client that implements other protocols as plugins. If and when such a thing arrives, I’m setting it to be my default.