Single-threaded concurrency was never the problem to begin with and it's used everywhere, in specific forms. Even C has it... for instance in your example printf formats to a buffer and writes at some later time, which is concurrent in that the next numbers can be created and formatted before the first is printf operation is complete.

But the reason almost nobody uses any of the general forms of this concurrency (coroutines, fibers) is because the general case of this is as useless as it is easy to do. For instance there are plenty of C coroutine libraries (including Russ' libtask) and they work fine. The reason nobody uses them is the because the situations where this is called for are precious few.

The other day on reddit Ian of gccgo even butted into a conversation about 1:1 threads vs m:n threads, but could not muster up an answer as to what conditions m:n threads (ala goroutines) would be called for. Simultaneous execution (threads for example) was always the hard part and any talk about concurrency that isn't about simultaneous execution is just tilting at windmills.