They could have even take inspiration, imagine: typed, functional, monadic init files - "systemd: avoid success at all costs" :)
Lua would have been a good candidate: memory safe, coroutine to structure the code in a concurrent-ish way if needed, and you have a configuration file parser for free (just use Lua as the configuration language, it was its very first purpose after all).
From system programming solely: libguestfs, Xen API, liquidsoap, Unison, MirageOS, google drive on fuse, 0install.
OCaml concurrency is great, and Lwt [1] would be exactly what systemd need: run daemon and poll the answer concurrently.
Ada, C++ and D would have been another great choices.
I wouldn't expect CPU-bound multithreading to be much of a concern in an init/rc system. And resource thriftiness and predictability would likely be a bigger concern.