The issue the OP is addressing covers your comment quite well. Async continuations, coroutines, and fibers can all race ahead and exhaust memory/DB connections/network sockets, or just saturate the IO subsystem. In a multi-step process this can easily lead to starvation in downstream steps.

The point of structuring an application this way is to prevent any one stage of your pipeline from consuming too many resources or stalling other stages (or just hammering some other system with requests that will inevitably timeout).

I don’t see how your comment about fiber support in Java applies.