Earlier quoted context omitted.
It's definitely not "dead". As commented above me, there have been massive improvements. 20 years ago laptops had two hours of battery life. Today a MacBook has 20+ hours of battery life. Obviously many components have become more efficient, but battery improvements have also contributed to that large increase in battery capacity.
The iBook G3 had a 45Wh battery which is only slightly less than a M2 air, and barely under half of the maximum you can take on a plane. Battery development has led to smaller, lighter and cheaper batteries in laptops but not really to an increase in capacity.
The 100Wh battery limit means they have little incentive to further improve battery technology. Any increased storage capacity wouldn't be legal to fly with, so all it could do is make the laptop very slightly thinner or lighter.