I have good answers for all the points you bring up so let me try to sort it out.
Grizz, by your theories, the D9 would be the *most* popular plane, since it is better in almost every way for the hit and run style compared to the P-51D, as is the La7. "Guns"? The P-51D has mediocre lethality.
The D9 is one of the most popular rides first of all. Imo the reason it isn't as popular as the P51D is because of its forward cockpit visibility. The 20mm's are hard to aim with for the average player especially with all the obstructions in the forward art. If HTC switched around the forward arts of the P51D and Fw190D9z I am convinced the D9 would take over as the most popular ride in the MA. And also on top of that, 50 cals are easier to aim with even if they aren't as lethal, but once a player learns how to aim 6 50 cals is extremely potent.
The P-47N, Typhoon, and Ta-152 (361mph OTD, massive firepower) should also be "up there" if your theory was correct. They are not. In reality, out of those planes only the P-51D and La7 are amongst the "most popular", and the other two of the "most popular" are the 344mph OTD SpitXVI, and the 325mph OTD N1K.
Typhoon: Haha Bnz, the Typhoon is a top 5 killer in the MA, I would consider that part of the 'most popular' group and in congruence with my theory.
P47N: This plane just 'feels' slow and actually is quite slow without its wep (which only lasts 5 minutes). It takes forever to climb up to a reasonable altitude and accelerates like a dog.
Ta152: Wobbly handling, difficult ballistics, and its obstructed german forward view (like the D9) deter average pilots from flying it. Imo you need a set of rudder pedals to have any chance of flying this plane with any sort of success.
I feel the average pilot usually makes a choice what he wants to do. Either A) SPEED AND GUNS which are the planes I have mentioned and are the most popular of that mentality or B) TURN AND GUNS. The absolute best turners with Gunz that also have decent enough speed to compete in the LW? Spits and N1K2s.
The airplane choices in AHII aren't about speed alone, or likely any other rational basis. They do not reflect the worth of a plane, or the ease/difficulty of getting kills in it. IMO, it would be foolish to determine ENY on a "popularity contest" basis. I mean, up to 50% of sorties may be flown by pilots who don't have any firm idea of the relative performance of their ride vs. the rest of the set. ("I thought the P-51 was the fastest...").
I don't think it should be based 100% on popularity alone but I do think that popularity should be playing a larger roll in determining Eny. With the P51D consistently ranking as the #1 Killer in the MA I have a hard time seeing how it can be Eny 8 and not 5.
There would be something positive coming out out Enying the popular rides more stringently though. Players would be forced to try other less popular planes. It would diversify the arena more and teach the majority of players about what they are missing in the rest of the plane set.