The first is not always possible (not all systems are elastic or can be, due to money/resources), and the second is not really a way to handle back-pressure. You could have back-pressure in the most optimized system.
> You could have back-pressure in the most optimized system.
Most batch processing systems. But you don't want back pressure in interactive or real-time systems, like graphics, gaming, audio, cars, planes, or even just real time collaboration systems (e.g. Figma). In all these cases back pressure is to be avoided.