This is part of being a sandbox and happens in most games I knew that are remotely similar. There's always some more 'effective' or popular tools than others. The only way to completely regulate it would be to remove large parts of the sandbox aspect.
Let's just look at the old MW arena. The popular LW muscle machines were totally absent, Spit VIII and Typhoon were perked. To many, the planeset was much more balanced than the LW one.
But still it was "the same half dozen planes all the time". In MW tour 100, the top 6 (out of 36 available) fighters made 52% of all fighter vs fighter kills. The top three shared 35% of all kills between them. A huge part of the planeset played only a minor role.
I dont have any numbers to back up my thoughts, your chart shows P51, Yak3, Spit16, D9, Nik, and La as the top 6. Seems to me we use to see a lot more 109 other than the K4, F4s, F6F, Spit8, and 5s, Jugs other than the M, and on base defense Hurris, IL2, Brewster and Zeros at sea.
All that diversity is gone. It is rare that you find these other planes today. Sure you can fly any plane you want, but to catch most other players you need the speed demons which only makes things worst. So your choice is to fly a speed demon, or log off. Neither choice can be very good for business.