Should I build one system or system of systems in order to fulfill enterprise needs?

I had that argue with one of the people that I’m working with. He claim that in order to create software solution to fulfill enterprise core business needs it’s better to create just one system (combine from several sub-systems) while I arguer that I think it better to split the solution to several systems. Every system should be responsible for certain enterprise need and probably export services to other systems as well. All of that systems, in the end of the day, serve as one system from the user point of view (it some sort of expansion of the Presentation-Abstract-Controller (PAC) architecture pattern). Part for arguing that following my line will result in more complex development process (which I don’t agree) I can’t figure out what are his claims against System Of Systems (SOS) approach. But I just want to mention the points that I mentions on behalf of SOS. I hope I’ll hear some claims against SOS from my blog readers….

1 – Splitting complicated problem to smaller and simpler parts is core idea in the software industry. So why not follow this idea and split our complex solution as set of independent solutions.

2 – Splitting the solution into systems enables us to create the global task (core business) in stages. Therefore that splitting enables less risk in development process.

3 – several systems with predefine interfaces which are black box one to each other enables changes in one system without touching other systems.

4 – the idea of creating several systems that serve as one system enable us to easily add new systems to the current system, with minimum chances to existing systems.