I looked up some of the planes and vehicles that Nashorn mentioned:
TA-152, 43 built, production started 1944
ME-163, 370 built, production started 1944
Tiger 2, 492 built, production started 1944
Wirblewind, 105 built, production started 1944
Ostwind, 44 built, production started 1944
F4u1c, 200 built, production started 1944
AR234, 210 built, production started 1944
P63, 2600 built, production started 1943
There were approximately twice as many P63s produced as the TA152, ME163, Tiger 2, Wirblewind, Ostwind, F4u1c, and AR234 combined.
I understand the argument that the Late War arena shouldn't be dominated by planes and vehicles that played a minimal part in the actual war. If that approach is taken, should wirbles be everywhere defending against planes? After all, only around 100 were built. What about ME163s defending the strats and hq? They only shot down a few planes during the actual war. Etc, etc, you get the idea.
I think the P63 clearly should be in the game at some point.