Lets talk about aeon gun, yes it is only 5 meter additional range, but think how much safer you could overcharge bricks or percies.
Now about mercy, if you perhaps did not notice that eaxh rts uses default priority system and in air unit category highest priority has torpedo bomber which basicly means that for 270 mass you can mecry anyone.
Now about czar, did you notice its insane hp bar and insane sam range? Id like you to notice that itw flying fortress that can brun bases or any other land t4 and it should help you win air.
Now about gc, you simply dont understand its strong points, as you noticed so far there is no unit that would win you a game without proper support, for gc it is t3 harb army, gc range helps you freely inflict damage on every army composition besides snipers, that means that you are forced to engage gc with your t3 units that have much higher cost than a harb and the same dps as a harb. Notice that snipers will not reallly help because they would have no shield since aeon army has a shiled damage based unit.