Any sufficiently advanced networked computer will have the potential for spam or malicious users. iMessage already sees this without being an open protocol; it's deeply-integrated nature makes it a prime vector for malware and 0-click spyware. Adversaries like NSO Group actively exploit this.
The goal isn't a more locked-down phone, it's transparent communications infrastructure that inherently resists attackers. Anything else is an imperfect solution that relies on trust more than mechanical security. If Apple wants to lead the way on that, they should do the world a favor and propose their own open SMS encryption standard. As it stands, their 'ours is better but we wont show you' approach is about as obvious as security theater gets.