While its true that it is the purpose of neither to incentivise side switching, it is undeniable that they do have a limited effect towards that end.
If it were up to me, I'd make three changes. Even if they don't necessarily incentivise side switching, they would certainly make things more "fair" when fighting against large numbers.
1) The upper ENY limit would be changed to 40 (i.e., you have to work even when hording). ENY 40 would kick in at 20% above current ENY cap numbers, and adjusted from that point.
What this does is make it near impossible to effectively steamroller bases with sheer weight of numbers, and ensures that when one side has overwhelming numbers, it will never have the advantage in quality of aircraft.
2) Drastically increase perk multiplier as numbers increase. The reason for this is two fold; when there is small but still noticeable disparity in numbers, the lower number side is more heavily rewarded for their work. Fighting 2 v1 is harder than fighting 2 separate 1v1 fights, and should be rewarded as such. Second, it will help counter the increasingly high ENY aircraft encountered as numbers increase, so that the low number side is still netting a reasonable amount of perks per kill.
3) increase the price drop on perk units as enemy numbers increase. While true that the low numbers side is already at a qualitative advantage, by the point the perk prices make a significant impact, they are already at an overwhelming quantitative disadvantage. 10 T-34/76's can swarm a Tiger II easily enough. A 262 can only cover so much ground, and only has so much ammunition; eventually, the C-47's and M3's will overwhelm even a group of defenders.
What this does is make the use of high perk units sustainable when at an extreme numbers disadvantage. If you're outnumbered by, say, 3:1 in total, you should be looking at around 50 perks for a Tiger II.