Those SMS/MMS responses are known as "fallback" texts, and in iMessage, they're sent in place of reaction messages to anyone who doesn't use iMessage. In Google's Messages 5.8, released on the Play Store to beta testers, 9to5Google spotted code that suggests Google is preparing similar "fallback" text, which would be sent to anyone who doesn't have RCS.

As 9to5Google points out, this could further the petty, green-versus-blue bubble war between Android and iOS users, as iMessage and Google Messages users could find it frustrating to receive fallback texts from each other. Plus, we don't know how long it will take before these text responses are available in RCS apps beyond Google Messages, like Verizon Messages or Samsung Messages. If those apps can only receive fallback texts, it could make messaging even more fragmented.