They won't. It's part of their "slowly-closed-source-all-of-Android" plan. The old messaging app used to be part of AOSP and you could read the source, how delightful that was.
Would be a good time for a contender to start the third OS. In a few years, more of the population would have interest enough that it might actually get traction.
Fucking A. I hate using Googles shitty messaging app, and maybe I would have more than 2 other rcs users in my contacts if the 3rd parties could implement it.