[1_player]

It would make pipes, the shell, and troubleshooting programs an order of magnitude more complex.

What is you want to inspect the ALT flow? How do you code logic that concurrently reads and writes data without blocking, deadlocks or decent performance? What's the terminating condition? And is it necessary at all? Pipes describe transformation of data, bidirectional pipes imply the transformation of the agent/process itself, not only the data, which flows in one direction.

Make it two orders of magnitude more complex.

I mean, it's possible, but unidirectional pipes are much easier to grok and pretty excellent already.