I think you need to consider the user interface here.
Someone with the phone number A texts a message to number B. Say the message is forwarded to number C. In order for the user with device C to respond to A, it would first be necessary for B to rewrite the message text to say that the message came from A. They then the user of C needs to make sure to manually send the message to A. Now, depending on how savvy the user of A happens to be, they might not find it completely disorienting. If you were to attempt to send to both A and B via a group text that might help some. But then you'll also be getting extra copies of your messages fed back to you from B. Unless the forwarding on B is also smart enough to filter them. And then there is still the problem that in future conversations, the user of A might just send messages directly to C, bypassing the other forwarding recipients of B, defeating the point of having a shared point of contact.
So forwarding has considerable awkwardness and room for error.
Someone with the phone number A texts a message to number B. Say the message is forwarded to number C. In order for the user with device C to respond to A, it would first be necessary for B to rewrite the message text to say that the message came from A. They then the user of C needs to make sure to manually send the message to A. Now, depending on how savvy the user of A happens to be, they might not find it completely disorienting. If you were to attempt to send to both A and B via a group text that might help some. But then you'll also be getting extra copies of your messages fed back to you from B. Unless the forwarding on B is also smart enough to filter them. And then there is still the problem that in future conversations, the user of A might just send messages directly to C, bypassing the other forwarding recipients of B, defeating the point of having a shared point of contact.
So forwarding has considerable awkwardness and room for error.