If they were functionally equivalent, why signal a difference? What's the benefit, you think? I'm not saying that they definitely would signal a difference, but in this hypothetical, if the color difference remained, it would be obvious why.
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))
Equivalent does not mean the same. Sending an actual SMS can cost extra.
Once again, I'm engaging with a hypothetical. Indeed this is how it would play out in reality.