Why introduce a dynamic scripting layer if you can dependency-inject a crate, compile your custom version, and replace whatever is running just as if you'd be restarting the server, picking up where it left because all the important stuff is ACID? (Or at least I hope that's how lemmy works, never actually had a look at the code).
Certainly less development effort, gets as cheap as putting the function to be customised in a lazy static. The difficult part is the API that goes along with it, providing a stable interface to plugin writers because if you don't have that you can just as well tell people to patch the function directly. And at the same time lemmy's version number starts with 0 which doesn't exactly sound compatible with API stability, and bugs need fixing. As such it comes down to the same thing: If people want a plugin system, they'll have to contribute as dessalines and nutomic seem to be perfectly busy keeping the ship afloat.