Except RCS offers basically none of those benefits. Encryption: no. Centralization: yes. It's a little decentralized because you can go through your mobile company and skip others if your recipient is also on that provider, but it's still using a relatively small set of centralized servers, and given the number of times people have demonstrated successful attacks on those servers, it's likely not private.
You want decentralized, encrypted, text communication with support for rich content, images, etc? Good news. We have that. It's called email with PGP. You can use WiFi or cellular to do that. RCS is no more secure than SMS and likely less useful or secure than most centralized chat apps.