The complexity of an async lib/framework such as i.e. projectreactor stems from that your code stops being linear. And so is the dataflow. This is the core of the issue. You trade simplicity for performance. Using futures you can still write simple linear code. The problem is that thread based execution doesn’t scale well when the threads are mostly waiting for io, such as i.e. an http request. That is, it was a problem until virtual threads arrived.

You don’t really need anything, they just make forking and subsequent joining+exception handling easier. You can just use the decades old thread api over virtual threads as is if you wish, but this “structured concurrency” concept, similar to goto vs structured control flow will make it much more productive and easier to reason about.