I don't mean that. I mean that eventually the business is going to need some feature that requires breaking the acyclic rule.

Ah, you don't mean enforce a novice making a mistake, you mean ensure from a design purity perspective?

I don't think its true that you need requests to flow both ways. For example, if a downstream API needs more context from an upstream one, one solution is to pass that data down as a parameter. You don't need to allow the downstream services to independently loop back to gather more info.