Maybe the "hyping this month" thing was glib but you cannot overestimate the effect the American culture has on the thing's usage. There is no other reasonable explanation for the P-51's popularity vs. planes that are similar in capacity.
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I've flown the Pony and its competition a good deal, and there is just absolutely no advantage to the Pony I can see that would keep a P-51 "ace" from switching to something else and being just as, if not more effective.
Just a thought, but do you think perhaps the perspective you're missing is that of the large numbers of players who
haven't flown the Pony or anything else a great deal? The relative effectiveness of two planes in the hands of an expert isn't the same as the relative effectiveness of the same two planes in the hands of a newbie, or even a pilot of moderate experience who is unfamiliar with that particular plane.
Also, I think range and carrying capacity are more important than you seem to be allowing for. What is happening in that respect is that as ENY rises, you are given a choice: carry lots of ords a long way to pork bases and suffer a penalty in dogfighting, or take a plane that can fight better but forget the porking, or at least do less of it. That might explain why the F4U-1D now has a lower ENY than the 1A model when the latter is, in the game at least, equal or superior in every respect except ord loadout.
I definitely agree with you about the "popularity" aspect of ENY working at cross purposes to its stated purposes. I like some of these changes but the system still isn't doing what it's supposed to do very well.