I think the reason was to save costs too? Also these laptops still looked fresh compared with thick PC "laptops", even now they look quite decent, but it might be just marketing strategy what will force Apple to change at least MB's design. Steve was going for glass and aluminum, and macbook is not aluminum at all. Besides I will not expect very radical design change. For example white imac and aluminum is quite similar design. If current Imac were thicker you couldn't see that much difference. I think it is reasonable to expect more rounded MBP's might be sloped style, like MBA? Thinner front and thicker back? The question is what stays from current design? HD, DVD drive? Can you reasonably shrink motherboard? Would it be OK with customers if you put embedded battery like in MBA? If you can answer these questions I can tell how MBP will look like. From all the updates I would value weight adjustment. I wouldn't mind shaving off at least a pound...
I think that the next macbook redesign will look close to the current mba but thicker.