Just to make sure we are on the same page before I begin. These are "stories you can do with gods, but not archfiends"
Coming to the prime and being killed by adventurers is a story you can possibly tell with both evil gods and archfiends. This seems incredibly interchangeable so it doesn't fulfill the point.
Now, I can already feel your accusations, so let me clarify, again. I am not saying this is a bad idea. I'm not saying you are wrong that gods rarely come to the prime. I'm not saying this is a bad story. I'm not saying you must bow down before my "one true way" story. I'm saying, and only saying, that I could use an evil god in the exact same plot of coming to the mortal realms, of players hunting down an artifact needed to truly kill the god, and doing so. I could also tell the story of archfiends being completely unable to form upon this mortal coil. these stories are interchangeable. So they do not support your claim.
No, again, these are easily interchangeable. I can easily imagine an evil god who is not widely worshipped, whose followers are secretive members of a cult and the adventure is about unveiling them. I've even got a DnD example in Vecna from Exandria and Critical Role.
Reversing that again to archfiends being openly and even proudly worshipped... I've actually done that in my campaign already. Do to some inter-party shenanigans, alliances between party members, and some major plot points, a potential future for my homebrew world involved elves openly worshiping the new Lady of the Fifth, The Fallen Huntress Tana. She offered them bloody vengeance against the mindflayers for horrific crimes against the elven people, and was friends with the party member who was basically an elven Arthur, rebuilding the empire and bringing it back to glory... and openly stating that they would allow legal worship of the devils.
So, again, secretive cult worship of an evil being, or open to the public worship of an evil being. I can do that with either Archfiends or Evil gods, so these are not stories that can only be told with one or the other.
Sorry, I can easily picture an Evil God of contracts, who treats his worshippers in a Pax Romana style of transactional worship. I can only grab part of the article (I heard about this in a video) but to give this context from the Britannica "Yet Roman religion was based not on divine grace but instead on mutual trust (fides) between god and man. The object of Roman religion was to secure the cooperation, benevolence, and “peace” of the gods (pax deorum)."
So, under this style of religion, a contract between man and god is expected.
This isn't a difference at all. There is no difference between a preist of a god or a cultist of a demon, and gods have sent angels plenty of times to give quests, so sending a fiend is the exact same.
They don't typically, but is there any reason they can't? I can easily imagine an evil god attempting to spread their influence and weaken their rivals by putting their own influences and iconography into the worship of another god. And mocking other god's beliefs and practices is easily done as well. In a way, we do it all the time.
And, flipping it, I see no reason that Demonic Cultists can't do forced conversions or murder. They are pretty good at both, actually. So, again, I don't see this as a story that is impossible to tell with the opposite type of being. It isn't typical, I will give you that, but an atypical story doesn't mean it can't be done or that it can't be done well.
And, since I've gone through a few of these, I'll go ahead and clarify again. I'm not saying your ideas are bad. I'm not saying these ideas are wrong. I'm not trying to convert you to the "one true way". I'm not trying to "gotcha". I'm honestly looking at each of these examples and asking "is it possible for me to tell this story with the opposite type of being" and each time so far... yes, it is.
Not sure if this is meant to be a seperate example but "lesser of two evils" is also possible to do with both Archfiends and with Gods. You don't need to pit them against each other to pull that off. Internal conflicts between the two can lead to the same story.
So, you can easily tell a story where the gods are not a natural part of the world. Where they come from beyond our realm to play their games with us mortals like chess pieces and an evil god is disruptive and harmful to the very fabric of reality in the same way as your archfiend example. I've actually seen it done, though the name of the work escapes me at the moment.
You can also have it where the fiends ARE part of the natural order. Japanese Yokai embody this entirely. They are evil spirits, but they are a natural part of the world. A part of the world that desires to harm you, that enjoys harming you, but the world isn't a kind place.
In fact, one of your earlier ideas in this thread (I think it was you) inspired a dark gothic world in my head where the fiends are natural and the gods are unnatural, and the natural state of the world is one of darkness, blood and terror, and the unnatural gods are preventing this state of primal darkness.
So, once more, I find this unconvincing. I can do either story with either type of being.
Which has always been a bizarre story to me. I would have to admit though, the "archfiend is trying to become a god" story is one that can't be swapped like I've been doing...
Except, the larger archetype of the story is "powerful evil being is trying to become more powerful, and we must stop them". I bring this up, because the famous 2e Vecna three-part adventure involves Vecna (a god) tricking Iuz (a demigod/god) to entering his domain, so that he could consume his power and become an even greater and more terrifying god in his bid for the control of all of reality by conquering Sigil.
So, while I fully acknowledge that the specific story of "non-god being trying to become a god" isn't able to work without one side being a god and the other not, the larger trope of "evil power-up ritual we must stop" does still work with either case.
But hey, we got one.
All of this can be done with Evil Gods. Endless war between Magbuliyet and Gruumsh is very similar in style to the Blood War. Coups can happen to gods. Happens pretty regularly in various fictions.
Wow, a lot to unpack here.
Petionersin a god's domain want to be there? Okay. But they are an evil god right, so it is still terrible and likely torturous place. And Archfiends can have the same story. I actually had a player recently who was playing a Hell Knight, they wanted to go to the Nine Hells when they died, so they could become an eternal soldier fighting for the cause of proctecting (conquering) the multiverse. Many cultists want to go to the hells or abyss because they think they will come out on top, and they are often proven wrong... but there is no reason that same thing can't be true for an evil god's domain.
Souls that don't belong in the evil realm of the evil god/Archfiend? Yep, that story can and has been told with both types of beings. That is the entire trope behind unwilling human sacrifices after all. And adventurers can go to rescue from either place.
Also, I don't even agree with your final point. Gods also have a lot of use for living beings and use them as proxies all the time. And living beings can be held captive or in enforced servitude by evil gods or by archfiends. No difference in that story.
And "far more often" tells us that these stories can be told with either archfiends or Evil Gods. So, no difference here.
These are all excellent plots. But all of them could be switched and told from the other side. So, again, to reiterate, the question asked was "What story can you tell with an Evil God that you cannot tell with an Archfiend?" and the only one you provided is an archfiend trying to become an evil god.
Again, I'm not saying these are bad plots. I'm not saying that they are terrible ideas. I'm not saying you are a bad person for having come up with them, or that there is a "one true way" to tell these stories. I'm saying that I can take every single one of these stories and make it work with the other type of being. Except, specifically, the "Archfiend trying to become an Evil God". Which, technically, I could do "Evil god tries to become an Archfiend" but due to the re-defining that would need to happen, it would likely not work out as well, and I am willing to concede that plot. All the others were interchangeable.