You don't have to own everything. Base product is already playable full game. I don't have any addons for Railworks yet, but I play it from time to time, and it's fun. Same thing with FSX - I buy planes I like, and sceneries I want to have.
I'm very surprised ED haven't released additional DLC yet - I'm talking about campaigns and maps. I'd gladly pay ED for example 20 or 30 bucks for a quality Afghanistan, Iraq or Korea terrain compatible with both DCS modules.