I often ask analysts who are writing requirements documents do not create any "subsystems" or "services" just for requirement grouping purpose. I tell them such a thing makes undesirable design constraints, confuses stakeholders (especially business), and this is generally not a very useful approach. But from the other side, they are doing it anyway.
So, if you cannot break the revolution - you should lead it. I'll share my personal experience and knowledge about functional decomposition and the consequences of such decisions. Besides, I'll give you some advices and questions you shall answer before the creation of a new "user interaction subsystem".