Why is a very lightweight 15" laptop out of the realm of possibility? It's not like displays today are inherently heavy. If OLED panels become viable for notebooks, they weigh nearly nothing, but even with existing LCDs with LED backlighting, those don't weigh much either. They do need to pay attention to making the case rigid across a larger area, but that ought to be possible without adding too much weight given modern materials. The LG Gram 15 only weighs 1.1kg, for instance.