Unless I am gravely mistaken the goal in question is to, more or less, even the fighting chances of all players so that as many players as possible can have fun in the game that they all pay for.
Now, making plane factories that can be destroyed to remove that plane type from the enemy does not accomplish this. Quite the contrary, it worsens the situation as the country with high numbers will have a significantly easier time both destroying the lower number country's aircraft factories and resupplying their own aircraft factories. The end result is that the outnumbered country(s) end up defending against La-7s and P-51Ds using Spit Is and C.202s. The situation becomes more lopsided than had nothing been done.
Personally I've always liked the factory idea, but it does not work in the context of the MA. The variant I had thought up was a neutral country that owned the factories and sold the the Bish/Knit/Rooks. If the factory was destroyed all sides lost access to it. However that does not work in conjunction with the ENY limiter.