There is nothing wrong with doubt but in the end if the day we have to look at the technical aspect to ascertain feasibility.
I can't rely on doubt as much as I can rely on hardware in short
Well his doubt did come from hardware feasibility.
It's safe to assume that since the screen isn't the only component that draws a lot of power, a shift to IGZO would make it possible to have a smaller battery but probably not enough for it to leave space for a discrete graphics card and its cooling system. Even it if it did you'd have to consider that the discrete graphics card would itself draw more power, requiring more juice and canceling the effect of the shift to an IGZO display. So in the end it would end up with a sub-par battery life which isn't something Apple would like.
You'd also have to consider the price. It would be very close to the 15" rMBP's. If cMBPs were discontinued now there would be a huge price gap between the MBAs and the 15" rMBP that would have to be filled by something. I think there's a market for a computer with a better display/CPU than the MBA but for a price that's below the 15" rMBP's.
And finally, if they were to put IGZO displays in the 13" rMBP, why wouldn't they have put them in the 15" as well? That would have allowed a better battery life. Either they're too hard to produce in large quantities or it's too pricey to do so. The fact that they would be 13" instead of 15" wouldn't change that.