Given the amount of SMS spam and the lack of such on iMessage, I am, at this point, ready for iMessage-only life. The few friends I had who were not iPhone users, I gifted them iPhones. So far none switched back, and I get to ignore all non-iMessage incoming messages (and thus 100% of SMS spam)
iOS has features for SMS spam blocking, but all of the proactive ones require the user opt-in (not enabled-by-default).
There are 3rd party block list apps, there is an OS setting to trust the filtering of your cell provider, and you can report the sender/content of a message directly to Apple for the first message from a new sender.
You should be aware of the social impact this makes. iMessage is already partitioning Android and iPhone users socially. These products just make it worse and will exclude people from social groupa even more. There's also that lower income individuals who can't afford an iPhone and use Android devices for cost related reasons would lose out on event planning.