Now see what moding does for a game and company!

[Codex]

Well then compare to flight sims then ...

FSX, while it can be modded for free (I think via an SDK), its the paid "mods" that also see FSX still being sold today. Hell I saw MS Flight Sim 2004 on the selves at my local JBHiFi store, I asked the store guy if anyone is still buying it, he said the stock on the shelf right now was just brought in to replace the copies that have been selling over the past year.

X-Plane, sales are still strong. Any there are heaps of mods for it.

And to get back to the OP, both my sons just bought ARMAII off steam just to play that mod. You know from what I've seen from watching my sons play, ARMA II is a FPS sim, there are many aspects to it that set it apart from CoD (Call Of Duty), like getting shot once will kill you or severely wound you. If you're wounded, there are no health packs to pick up, and blood loss becomes an issue.