what would it look like if you didn't need concurrency at all - would simply having a step by step process enough e.g using DAGs

What business systems don't use concurrency in some form? I can only think of the simplest data processing tasks written for batch processing. But even every embedded system I've ever developed or worked on used concurrency. Though for older systems this was often hand rolled, and as error prone as you might expect. For newer systems (developed this century), it was often done using a task system baked into the embedded RTOS.