So, DCS is not limited to "modern" aircraft. Its called Digital Combat Simulator. They are setting this up so that you can play any time frame of combat whether its WW1, or 2, Korean or whatever time frame you want.
Also, going by your logic, there is already an F16 simulator so why do we need another one? They do the WW2 and older aircraft first because it's 1 easier to get the info they need to make a DCS level aircraft and 2 because it brings in the extra money they need to continue the development of the game so that you can get the modern aircraft you're talking about.