For me it comes down to simplicity of the sharing model. With async you can basically ignore sharing and write stuff that works against the same data structures because you know they’re mutually exclusive. Once you move stuff into separate threads you need to be a lot more careful, using Arc and Mutex et al, or devise channel semantics. I tend to only use threads when I never want a future interfering with the scheduling of another future, either when one is really long or when low latency is absolutely critical. One example recently was I had very low latency network stuff happening on one thread and it was sending messages to a TUI thread. I absolutely didn’t want the networking code ever blocked by the TUI, even for short times.

I find it fascinating that LWT have come so far that this is a legitimate question. There was a time when async << threads << processes. But a lot of heroic good work went into bringing about this wonderful age.