Yeah, "async is a niche requirement" has long seemed weird to me. Most programs with any degree of interactivity quickly come to a point where they want more than one thing to be happening at once, and they spend some time waiting on external inputs -- whether those are network packets or USB events or timers.

Purely synchronous code only suffices for that narrow and most boring category of programs that know all their inputs up-front and don't accept any other inputs or interactions at all.