muskrat
Adding encryption for any existing app is a big lift. There is an huge infrastructure lift if you are going to build the full stack yourself. A smaller life if you choose to tap into someone else's infrastructure, if it exists, and it will come at a cost. Likely a cost that a small FOSS Project could afford, especially if theirs usage picked up quickly without charging a fee.
RCS, as a specific example, interfaces with cellular/mobile carriers directly so you can contact those people on T-Mobile, AT&T, Verizon, etc. Each carrier has it's own infrastructure that registers users and then opens up to the wider RCS network. Essentially, this is a federation of sorts. Now, QUIK, for example, could skip creating their own stack and use Twilio's stack, but that isn't free. Each message sent will have a Twilio fee to cover their costs and profits, and it will also pick up a carrier charge. QUIK cannot eat that, and they would have to charge a fee to their users for this "premium" feature.
Source: I'm a developer who works directly with Twilio integrations into applications for SMS solutions, who researched RCS implementations using Twilio's Business RCS offerings. https://www.twilio.com/docs/rcs