Over in another forum, it was purported by several that the Mitsubishi A6M was an obsolete design by the last half of the war. Specifically, they claimed it was at the end of it's design limits. I'm not all that sure the Zero was at it's design limits.
The ailerons were delightful until high speed. They could have been changed to ailerons and elevators better-suited to higher speeds and the control system mechanical advantage could also have been reworked. Just because the Sakae 31 was near it's limit does NOT mean another engine that made more power could not have been made to work. The Ha-112-II Kasei was lighter than the Sakae 31, made 1,500 HP, was reliable, and it worked quite well in the Ki-100 that originally had a much smaller overall diameter engine in it.
All of the above said, I think the Zero WAS at or near it's limit with the existing engine and design. Everything above is meant to suggest design changes to the airframe that enhance either power or higher speeds.
A lot of folks thought the P-40 was a dog. Then the XP-40Q came along and showed what could be done with design changes. That we didn't buy it is our own fault as it would have represented a major step forward everywhere the P-40 was in actual service. Wherever it went (replacing a P-40) the XP-40Q would have been a quantum leap in performance. And we used a LOT of P-40s, right to the end of the war. I completely disagree with the guys who said it wasn't needed and didn't fulfill any role. It could have replaced older P-40s that were still serving and was a MUCH better airplane than ANY of the standard P-40s. It STILL had the same range of a standard P-40, and would give a P-51 hell all day long. That's an improvement over an existing P-40, no matter how you slice it ... and we were were USING P-40s on the last day of the war.
The Fw 190 was a very good aircraft. The Ta 152, developed from the Fw 190 was quite a bit better, even if too late to be of much use.
The Spitfire got a major face lift with the change to the Griffon engine.
The P-51 started out as a good plane and got a major face lift with the change to the 2-stage Merlin.
There is nothing saying the Zero could not have been reworked in the same manner, especially with the Ha-112 II Kasei engine, and that even the arifoil cold have been reworked to be a higher-speed aircraft, keeping a LOT of the original design. That they didn't DO it is probably more a testament to the effects of continual attacks rather than inability to rework the A6M design. Would it have taken effort? Yes. But much LESS effort than was expended on things like the Kyushu J7W1 Shinden that made only one or two wartime flights, and none in combat. There are numerous others that come to mind as useless wastes of effort that might have been better put to use with a reworked Zero airframe.
Any opinions to offer?
I'll remind everyone that ALL the big fighters at the end of the war had at least some sort of "makeover" as an upgrade; Spitfire, P-51, P-47, certainly the Fw 190 (into D and Ta 152 models), Yak-3 / 9, and even the Bf 109 (late G / K). What would stop the Japanese from doping the same?