Well next Iris Pro will be faster than the 750M but still nowhere near an 850M. If they sold 850M notebooks now, how can they explain removing the dGPU option and only offering slower options then last generation. If they leave the 750M in they can state on their homepage it is faster than the last gen and all those non tech people of whom there are many will be none the wiser and not complain. Apple already had the reputation of being bad for gaming, people will just think it is more of the same.
The MBP used to stack up well against similarly slim mobile Windows notebooks in performance also but now they don't seem to care anymore. A 970M cbautis2 mentions is 4-5 times faster but that is a different class, still you can have it in fairly slim notebooks not even that expensive (MSI GS70 with 965M is great for gaming and price). Yet a 850M with similar TDP as the 750M is still twice as fast and not even that is in the cards with Apple.