I consider that many showed you the exact opposite of what you "showed" us. The evil gods are in fact, far more meaningful than archdevils and demons as the later can grant spells only if optional rules are used. As soon as you use an optional rule, you homebrew.
What edition are you talking about? Who are you supposing gets to decide whether or not an optional rule in 3E is used? What is your measure of "meaningful"?
I can tell you that
in my play of AD&D I did not regard evil gods as more, or less, meaningful than Archdevils. As a GM I would use NPCs who were priests of evil gods. And I would use NPCs who were priests of archdevils and demon lords. Their role and purpose was much the same.
No doubt it is possible to do what you and
@Maxperson seem to have done, which is to imagine worlds for D&D play where the difference between "archfiends" and evil gods goes very deep. But the D&D rules, and the D&D published materials, have never mandated that D&D worlds be like that. And neither FR nor GH were like that in the 1980s (see, again, Dragon 91 for FR, and module T1 and the City of GH boxed set for GH).
Problem is that godhood can change from world to world and even editions to edition.
This is a "problem" only for those who want to assert that clerics of devils and demons have never been typical! It's not a problem for me.
When I use Erythnul the Many, or Vaprak the Destroyer, as figures of worship in my games, I don't worry about where they sit on some metaphysical scale of godhood as compared to Demogorgon and Orcus; doubly not as far as a scale that was not invented until the 3E DDG was published, given that my use of these beings in my game predates that publication by nearly two decades.
Orcus was a mortal man once, so was Kelemvor. Both are now far more powerful than that. But the difference between what they became doesn't seem to exist unless you homebrew it to exist.
So Orcus was a primordial at some point.. When he died, he became Tenebrous as of 4ed and got reinstated as Orcus...
In 1ed he was just a demon prince and it was just assumed that demons would just spring into existence in the abyss. At some point, there were only 6 Demon type VI in exsitence (Balor etc...) MM1 1ed, p19.
In the Realms, Orcus started as mortal. In any other setting, he sprang into existence in the abyss. Spontaneously existing or, in 4ed, is a corrupted primordials.
It's more likely he started out as a mortal who died and became a larva (since that's assumed to be true for all evil mortals) than just appearing fully-formed one day.
In 2e there was just one Orcus and he was the same for all D&D settings, every entity was.
This is the first time I've learned that the FR version of Orcus has him beginning life as a mortal.
In the AD&D MM Orcus is presented as a demon prince, but nothing is said of his origin. (There is no "assumption" that demons would just spring into existence in the Abyss. Ed Greenwood was not going against any such assumption if/when he wrote up the mortal origin for Orcus in the FR.)
In AD&D it's also not true that all evil mortals become larva. From the MM (pp 17, 23, 59):
Those dead which go to the 666 layers of the demonic abyss become manes.
The lemures are the form which the dead whom inhabit the Nine Hells are put in.
The larvae are the most selfishly evil of all souls who sink to lower planes after death. They abide in the gloom of Hades, controlled by the night hags.
Does this mean that the souls of devout worshippers of Asmodeus who are also among the most selfishly evil pass, upon death to Hades to be controlled by night hags as larvae? Or are those souls an exception to the proposition stated on p 59?
There is no canonical answer to this in the AD&D MM; nor can one be found in the DDG or (as best I recall) in the MotP. Maybe Planescape tried to clean all this up in some systematic fashion - I don't know - but that doesn't change what was the case in these earlier works.
It's the same thing when trying to argue that, because some 2nd ed materials like Planescape or Spelljammer assert that every entity is the same across the multiverse, that Asmodeus as presented in the Greyhawk City boxed set scenario To Slay a Hierarch must have all the same attributes as Asmodeus presented in some 2nd ed-era FR source that I've never heard of. Namely, it's just not true! Or rather - anyone can imagine that if they want, when they read those sources and use them in play. But it is not dictated by the sources themselves. Greyhawk City is an early 2nd ed AD&D product, produced before Planescape or Spelljammer. When it refers to Asmodeus I think it is clear that it is not picking up any baggage beyond the entry in the AD&D MM, which has him as ruler of the Nine Hells, and the descriptions in the published WoG material of the Horned Society as built around devil worship.
The general point is that later works don't change the facts about what is to be found in earlier texts.