That is not a very good excuse. Redesigning the logic board is a piece of cake. It is a different chip but that mostly affects the software driver. On the board it is the same memory interface, the same power rails, the same everything except for a slightly bigger chip to fit on it. Apple engineers with their money and time can do that easily and they get engineering samples way ahead of time to get that done.
There is a company with huge resources and definitely the biggest volume in 2000$+ notebook shipments from all. They have a reputation to defend. And they go almost three quarters of a year without a very significant upgrade - that is like not adding quad cores but sticking with dual cores -, just because the they are too lazy to fit a new chip on their logic board.
That is just not reason enough. I don't see how that is reasonable.
The reason they didn't drop the dGPU with the 750M might be that the over-clocked 650M was essentially exactly as fast as the current under-clocked 750M and thus slightly faster than the Iris Pro. It is close but still slower which would be all that is left in the mind of people absorbing the reviews in the press. Iris Pro 2 should be about 40% faster than 1 and thus slightly faster to quite a bit faster than the 750M and almost never slower.
Why did they include Iris Pro at all is the better question. It is a bit pointless. With the standard Quad Cores being the more reasonable options. Probably because they had everything set to drop the dGPU and waited to see the actual performance and decided against it very late, when it got clear that Intels drivers would not deliver enough of the theoretical performance which could put Iris Pro ahead of a 750M.
