I get what you're saying, but I think the issue with optional memory safety features is that it's hard to be sure you're using it in all the places and hard to maintain that when someone can add a new allocation in the future, etc. It's certainly doable, and maybe some static analysis tools out there can prove it's all okay.
Whereas with Rust, it's built from the ground up to prove exactly that, plus other things like no memory being shared between threads by accident etc. Rust makes it difficult and obvious to do the wrong thing, rather than that being the default.
I don't want to sound like I'm just correcting you for the sake of it, but it's actually important. Mastodon is the most popular right now, but Mastodon actually wasn't around at the beginning! Before that was StatusNet, and before that was identi.ca and laconi.ca
So those services already existed, they were the ones built for federation, and so Mastodon was started as another compatible implementation of an existing network protocol. All of that is to say that Mastodon didn't need to make the right choices at the beginning, and they have already benefitted from this kind of network dynamic! The system has already worked once!