I went with marginal because I didn't feel like going educate myself on the gritty details I've forgotten for someone saying something that patently retarded.

Even if it was physically effective to some extent, which as you say isn't guaranteed, the psychological aspect means it will always fail. Because the type of person to drink diet soda isn't fat on just soda, but will absolutely be the type to think swapping to diet is "enough." In fact, it will often trick them into thinking they have "done something" in their brain and thereby deserve a "little treat" for it.

Those mindsets are why nearly all diets fail in people, the brain is fiercely self protective to the body's detriment and will absolutely maintain its own homeostasis tricks subconsciously if you don't hold it to account.