The hardest bit is encryption, you can get a basic app or bot running trivially from scratch if you don’t care about that. People successfully wrote matrix apps for years before the Rust SDK existed, and there are plenty of other bits of the matrix ecosystem that don’t use it (mautrix/Beeper is mostly written in go!). The Rust SDK is nice to use and give a bunch of bonus features on top, so that’s why it’s popular.