In the spirit of "every non-trivial program will expand until ...", I think preemptively choosing async for anything much more complex than a throwaway script might be justified. In this case, the relevant thing isn't performance or expected number of concurrent users/connections, but whether the program is likely to become or include a non-trivial state machine. My primary influence on this topic is this post from @sunshowers: https://sunshowers.io/posts/nextest-and-tokio/