The way I look at it, Writable/Readable compete with Array[Byte], not with reactive streams, ZIO, akka-streams, monix, or FS2. Using FS2 doesn’t mean you don’t have Array[Byte]s in your program, and just because you’re using Reactive Streams doesn’t mean you won’t have Writable/Readable in your program.
They’re simply different tools for different jobs, and neither is really a substitute for the other.