because they can’t support the same functionality. apple would add something new, they would want no constraints to what they can do.

> apple would add something new

Once again, I'm engaging with a hypothetical. Indeed this is how it would play out in reality.

there's two doors through which the message can leave your phone... the blue door and the green door. (Ignoring the SMS tiny door)

If the feature sets of the blue and green door happen to be the same for a time, that doesn't mean a message sent through the green door is "actually" leaving through the blue... The doors are actually different (even if the feature sets are the same (for a time))

We were (originally) talking about a mystical magical timeline where the two standards converge. You can make another hypothetical, where they converge, but only temporarily, but that's a new flavor on the hypothetical that was first posed way way back where the thread started. I'm sick of repeating myself, so I'll leave the musing to those who are more invested.

Branding. It’s that simple.

> If they were functionally equivalent, why signal a difference?

Equivalent does not mean the same. Sending an actual SMS can cost extra.