After 15 moths of having my Rift, it still amazes me every time I use it. Higher resolution would be nice, but it isn't as terrible as the hard data would have you believe. The human brain is amazingly good at filling in the gaps. For example, when I get my eyesight tested every 6 months for my flight physical, I can see better with both eyes open (20/15 with corrective lenses) than each individual eye can resolve (20/20 corrected). This is relevant to a VR headset because each eye is presented with a different image, and your brain does the rest.
In other words, the displays on the Rift and the Vive are better than the sum of their parts. Obviously, this will get better with future generations of the tech, but higher resolution isn't going to help unless you have a PC that can run it. You need a good gaming rig right now to run DCS with the Rift/Vive.
6DoF is a necessity. Without it you no longer have natural head movement within the cockpit, and that is a huge part of the feeling that you are actually there in the cockpit.
Last night I spent about 3 hours flying IL2 BOS/BOM. Despite the resolution it was an outstanding experience. The dogfights were intense and to a degree, physical , with lots of twisting in my seat to look around, and craning of my neck. Thankfully we don't have to deal with G's as that would be exhausting after a short period of yanking and banking.
Here is a hypothetical scenario. Just imagine that the current generation VR headsets had actually been available for several years and ran well on the gaming rigs of the day. Then the 4K monitors become available and the display looks incredible. So, what would it mean to jump to 4K under this scenario? You would have to give up depth perception, the feeling of being in a real cockpit environment and natural 1:1 head tracking. You would gain a sharper prettier image, the ability to easily read reference materials and easy interaction with physical periferals such as the keyboard etc. That doesn't sound like a good trade to me.