Like my USB mouse working with any computer, I used to be able to pick up a phone and text anyone on the planet without having to check which app they're on. Sure there are the 'de facto' apps, but these vary by country and social group. The reason for the proliferation of the third party apps lies squarely on the proprietary, ckosed protocols by each of them and viral growth during the early days when the telcos were still figuring out data-based text and voice - players were playing up their features like 'security' and 'privacy' and creating the walled gardens as you mentioned. The current leaders grew due to rapid adoption and a person's social clout. Just try dating nowadays in Asia, where multiple texting apps reside on a typical phone. Don't kid yourself - the messages mentioned earlier serve only the marketing goals of each app and just lock you in with FOMO or the hassle of switching. The companies don't give a shit about your privacy, as long as they can monetize your data, feed and activity.

RCS, to me, wants to take us back to industry standardization, so any provider can follow the standard and immediately be connected to everyone, instead of having to deal with different platforms, protocols. An iphone should be able to communicate with an Android and all the flavours without degradation or the color of the bubbles. Sure, the current implementation of RCS is google's, but the standard is not. Hopefully, managed by a (neutral) standards body, the shortcomings people point out will be patched and adopted by the app developers. Desired featured will be folded into the standards and make their way into then apps. This, again to me, while slower, is preferable than being tied to the whims of a provider, e.g. Whatsapp, iMessage and telegram, does away with the market fragmentation (not competition) and gets rid off all the artificial bullshit like blue/ green bubbles, security lapses and image degredation between apps.

Just my 2c. Do with it as you will.