Almost every metal laptop I've used has been an absolute pain because of the bending. The sound of fans scraping against metal is absolutely horrible and has killed more than one macbook of mine over the years. I even had one where my touchpad would randomly click unless it was placed on a perfectly-flat surface like a desk (no laps allowed!) because of bends. I'd much prefer a stronger material to stand the test of time, even if it chips.
Aluminum bends and flexes. Magnesium alloys would shatter (if you could even apply enough force). The chassis chipping or flaking seems preferable to bending and flexing for the longevity of the internals.
I really don't see how carbon fibre would be in any way better than what Apple uses right now.