Even if you do it completely top down, it doesn't mean all cells need to update if you change something on the top, so you'll still profit from the dependency g

Even if you do it completely top down, it doesn't mean all cells need to update if you change something on the top, so you'll still profit from the dependency graph. And notebooks are mostly for exploration, and you don't really have a fixed order. You start getting the data, then you run the model, then you go back and change the data in the import... The order of your internal logic (or how you want to explain it to others) isn't necessarily linear, and of course you can just go back and write like a normal program but you lose the chain of changes that led you to the result (in this case the compromise is that your chain is restricted, like you said you can't define the same thing twice, but in exchange the notebook will provide with you the program order for free).