I think Rust is a perfect choice here. Considering the investments of the Linux kernel, AWS, Microsoft and so on, I think Rust is a future-proof bet.
That said it think the programming language is not everything. It seems to me that lemmy was written under the assumptions that there will be a lot more hosted instances that will fedrate but a lot of load seems to centralize on a handful of instances now (i.e. lemmy.world).
To support these it could make sense to rethink the system design to something that offers better support for high-load and high-availability scenarios.
https://en.m.wikipedia.org/wiki/Paradox_of_tolerance