[Brightcolours]

Personally, I couldn't care less about the complexity of a lens. In fact, given two lenses, a simple and a complex one, featuring the same IQ, I'd take the simpler one any day.

 

Also: why is that that a f2.8-4.0 lens is easier to design than a constant f4 lens? It doesn't seem to make much sense to me given the 2.8-4.0 will be brighter at best and the same at worst.

The reason is because the apparent aperture needs to stay constant when wide open. At 16mm it needs to be 16/4=4mm, at 70mm it needs to be 70/4=17.5mm. To achieve that one needs to use a few tricks which give one less freedom in design.