Your anecdotal lived experience is not representative of the entire population.
I personally have encountered at least a dozen spam iMessages (not SMS) in the past year, and several friends of mine have described the same experience. I googled iMessage spam and this was on the second page, just from last year: https://thisrupt.co/lifestyle/imessage-spam-not-thai-chana/ Feel free to research yourself to discover that it is in fact a widespread issue for many people, if not as widespread as it once was since the "Unknown sender" tab was introduced.
Regardless, SMS spam remains an issue, and on iOS, many users may not know the difference, as they're in the same app.
> And even if there were a spam problem, the risk is mostly on the upside anyway. It would only be an issue if iMessage got a reputation for flooding people with admonishments to take security seriously, purportedly from Apple.
You're missing the point. iMessage spam (though it does exist as I've shown above) is not the problem. The problem is iMessage doesn't have a good way to "verify" that messages that purport to be from Apple or anyone else truly are from a known and trusted sender. This deficiency is what enables iMessage spam, and creates the same potential for abuse with this new feature.