SMS is not the same as a chat service. SMS to many people's perception is just as sensitive as a phone call because it was around prior to smart phones, and has been at the basic phone/carrier level for far too long.
Aside from all these "quality control"/"carrier usage sniffing" app services hiding at the admin level of Android in the background, people think SMS is seen by only them because SMS doesn't count towards your smart phone's data usage. It's its own plan.
All that aside, I honestly don't see Android having a dedicated iMessage-esque feature standard to all devices. That's just not in the vision of Android and Google as a whole. Only Apple would push off a feature that kills all the other innovations by its own user Developer base to-date because they want to offer a central standard feature (and Apple has done it many times over iOS's lifespan)