Integrating carbon fiber into the platform can't be cheap. While Audi/Mercedes are busy integrating aluminum, BMW is one step ahead by moving to CFRP with the upcoming G platform.
By reducing the number of platforms, they're able to achieve economies of scale. BMW must have determined this was the only way to make CFRP economically viable at this time. Scott26 can't say this externally of course.
Enthusiasts will still have the shared platform with Toyota, so that's a positive sign. Assuming the next-generation 4er uses this shared platform also, we'll actually see even more differentiation between the 3er and 4er, which would actually be a good thing imo.