Engineers will deliberately break things up into subsystems on the board. Engineering is about making a safe and effective product in a timeframe (read: budget)

Engineers will deliberately break things up into subsystems on the board. Engineering is about making a safe and effective product in a timeframe (read: budget) suitable for the customer, it's not about perfection. You are allowed to approximate. You can see this on some PCBs where a decision has clearly been made to separate (say) two different digital subsystems, which could be more efficient if mingled, but is only feasible for their resources if designed and verified separately.