What we need is a local eny to prevent the whole side from concentrating in one area.
ENY is one of those things that seems not to work, except when compared to other alternatives. To get around the current system, one can switch sides, within time restrictions, or up a plane above the ENY. There are plenty of very good planes at 20 and above ENY.
How would a local ENY work? What about a massive raid deep over enemy fields? The planes affecting ENY would already be in the air. So, would the country being bombed be given a lower ENY locally? Or would the nearest fields of the mega-raiders be given a higher ENY? Would the first
X planes of the mega raid have lower ENY restrictions than the last planes to launch?
I believe the current system has evolved over time. The delay in switching countries is probably to prevent people jumping back and forth as 'spies.'
Whatever is proposed, there will be holes in the system.
To quote the Talking Heads: "Watch out, you might get what you're after."