Strictly speaking, Moore's original paper is not about transitors per chip but about transistors density for the least cost node. So first it's about transisto

Strictly speaking, Moore's original paper is not about transitors per chip but about transistors density for the least cost node. So first it's about transistor density, hence the relation to having smaller and smaller transistor. And also and very important, it's about cost. You could go 3D to increase the amount of transistors per area, but that requires more masks/operations and impact the yield, and increases cost. If in the end the cost per transistor increases, then it's a density increase but it's not advancing Moore law. In other words, Moore's law can be broken in two different ways: 1) we can't improve anymore the transistor density; 2) we can improve transistor density, but not in a cost effective way. If you want more you need to pay more (vs. getting more for less money when riding Moore's law).