There are some very discouraging fundamentals that are inherent to text messaging itself. Unlike email, where it's irrelevant to what email service you use, all email messages can be readily and seamlessly exchanged between all those different services (they all support the POP and IMAP protocols as a standard). But with text messaging, it's a very different environment. There's a mix of text messaging protocols where some are proprietary and selectively closed (Apple's iMessage protocol and the WhatsApp protocol being two of them) and some are proprietary but open for public usage. So in that messy mix of protocols, some text messaging apps can support some of them but given the proprietary licensing restrictions none of them can support all of them. The only two protocols that all text messaging services do continue to support are SMS and MMS. They remain as the only common link in the convoluted mess of different text messaging services, carriers, platforms, and apps. The drawback being while SMS and MMS are both open standards, they're also both very dated (going back to the 70's) so they have a lot of issues with today's technology. SMS being text-based only a minor issue, MMS involving media attachments a more significant one, and both are from a time when things like bandwidth was always limited.
So someone using Apple's iMessage app sending a text message to another user also using the iMessage app will always involve exchanges using the iMessage protocol (and through Apple's online servers) while that same user exchanging text messages with an Android user will involve SMS or MMS (again, SMS for text only, MMS when there's an attachment, with the added twist that all iMessage grouped messages are sent to any Android user as MMS.)
As for Google's RCS protocol, it's one of those proprietary but open licensing for any service to include standards, but after years of pushing it to be the Android default to replace SMS/MMS it's only now getting more widespread acceptance. But even if it does become the new Android default text messaging protocol, a major stumbling block is Apple a) refuses to allow other platforms/services to include support for iMessage protocol and b) refuses to include support for RCS in its iMessage app. Either one would be benefit for us, the consumer, but Apple is doing great financially maintaining its own wall-garden environment so there's no incentive to serve the public, it only needs to take care of its own user base. So RCS or not, we're all still very much dependent on SMS and MMS and there's no indication that's going to change in the near future.
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