You say this like Apple doesn't release OS updates. Why are you putting that as some arbitrary limiter to what Apple could do to protect its walled garden?
They don't usually remove features as important as iMessage from older iOS versions. I don't believe they push updates to the iPhone 7 and older anymore, so they'd be unable to use iMessage.
I have a 6sPlus, and messages work just fine, and it may not be iOS 17, but I recently-ish ran an update for its OS that Apple deliberately updated (which you just know must have been an important update). You can stop making stuff up now
