As you say, the thinness craze was enabled by lower-power CPUs that performed no worse than previous gens. They need smaller batteries and less extreme cooling

As you say, the thinness craze was enabled by lower-power CPUs that performed no worse than previous gens. They need smaller batteries and less extreme cooling which means smaller sizes. Alternatively, keep the same size and you can enlarge the battery. Either way, perf/watt gains have enabled that in a way that's visible to consumers. With a plugged in PC, that's not visible.