Plus, even if cooling between layers were figured out, we'll probably just end up with lots of transistors that we can't all power on at once, since we're alrea

Plus, even if cooling between layers were figured out, we'll probably just end up with lots of transistors that we can't all power on at once, since we're already at practical power draw limits for most devices. We don't just need to cram more transistors on a chip, we need those transistors to use proportionally less power (preferably even less than that).