[jayd16]

Knowing when execution will yield is useful when you want to hold onto a thread. If you run your GUI related async tasks on the GUI thread you don't have to worry about locks or multi threaded data structures. Only a single GUI operation will happen at a time.

If yields are implicit, you don't have enough control to really pull that off.

Maybe it's possible but I haven't seen a popular green threaded UI framework that let's you run tasks in background threads implicitly. If I need to call a bunch of code to explicitly parcel background work, that just ends up being async/await with less sugar.