This may shock you but I'm not looking for a confrontation or a fight. I'm looking for a discussion.
If you go back to the OP, you will see that I DIDN'T ask what to do with them. I'm not the OP. I just offered some insights. Are there examples of what you describe where the forces of evil are weaker and more numerous? Yes. As you said, Judeo-Christian and Muslim practices do feature exactly that motif. Is it the most common trope in the world? I don't know for certain, but I don't think it is. And even if it is, it seems to me that in polytheistic frameworks like what you see in DnD you are far more likely to have forces of evil be equally powerful or more powerful.
See, I think that Evil Gods exist in DnD historically for two reasons. 1) To give us evil clerics 2) Because people were throwing all sorts of ideas at the wall to see what sticks. That's why we have dozens of overlapping deities, because people just made up a new force for their adventures constantly. This has led to a plethora of being mentioned once in a magazine and never again. Or maybe once in a magazine and then making a cameo in an adventure, then getting tied in with someone else's origin story.
But, if we ask the question "why do demons and devils exist in DnD" the answer is a bit self-obvious. Of course we have demons and devils to act as the incarnations of evil. That just makes sense. The third point I'm getting at, the disconnect comes in why the most powerful demons and devils aren't basically gods. And this is where polytheism ran face first into monotheism. It is very easy to see a being like Typhon as "Demon Prince" being the child of Gaia and Tartarus (easy to make the comparison, though it is inaccurate since Tartarus isn't evil in greek mythology) and was more than a match for the Olympians until Zues used his thunderbolts, one version of the myth having the entire pantheon flee to egypt in the face of Typhon. And "evil 'demon' that challenges the gods" is a very, very common trope in polytheistic religions. And while the Gods win, it isn't a curb stomp and it is often only the most powerful of the gods who win.
However, in DnD, "gods" are the highest tier of beings and far more powerful than the demons and devils, so it is goes that even a non-combatant god, like Hestia, would be able to destroy a demon prince with little effort... which just isn't how the mythologies around this would go. Heck, it is heavily implied in the story of Pirithous that a mortal man could kidnap a goddess (though putting Persephone in her proper context makes that laughable, Dread Persephone is far scarier than her well-known mythology implies. Minoan's man.)
Which rounds us back to the point that has been brought up a few times. Other than "because the game was built with this fact" what value do we get from "God" being a more powerful and higher order of being? Sure, we have the story of Orcus trying to become a God to be more powerful and do stuff... but we already have gods of undeath who have that power he seeks. If Orcus becoming a god of undeath was truly a world-threatening event, then the existing gods of undeath would be threatening the world and a much more serious threat. There doesn't seem to be a compelling reason not to make the Gods, Archdevils, Demon Princes (seriously, I want a better title for them), Archfey, Primal Spirits, ect all about the same power. I think it makes for a more compelling battlefield to get involved in if Orcus or Graz'zt CAN threaten the throne of Pelor or Amaunator, and the PCs can get powerful enough to tip that balance. As it stands, the only way to make that story happen is to have the Demon's first severely weaken the god, then attack. While the gods stand so far above everyone and everything else as to not really get involved except through agents.
Much more rested. Sorry if my reply was a bit more slappy than usual. It was not my intention.
A trope, is not
THE trope. It is one of many possible. The fun thing with D&D is that you can use one, two, three or gazillion and it doesn't matter one iota. There are ways to make them work together with one an other without even scratching your heads on how. Gygax found one, and I am sure you can find one by yourself without throwing one set of beings down the drain. So wheter you like it or not, it is a solution. Maybe not the solution you want but it is a solution nonetheless.
As for the bolded part.
What do we gain indeed. A lot of possibilities. Much more than if everything that has a name on it in the plane can be a god. By wanting to usurp gods' powers the demons and devils are effectively creating wars. They are numerous, endless but they are still bound by cosmological laws either given by the gods or by some other Overlord of the Gods (i.e. Ao in the FR and if you read the novels, you know that even Ao has an Overlord too.). And since that Overlord (i.e Ao) is present, then it makes sense that the Gods do have their own laws to obey... At which point do mortals get to interact with higher and higher beings? The Immortal set of BCEMI told us what it was all about.
In resumé for the Immortal Set which brought a lot of lights to our games. It does, afterall, litterally make you play gods (ok immortals, but in the OD&D it basically amount to same thing.)
The first set of immortal beings (and my views on how they should translate in D&D) are the
Initiate (Basically the quasi deities, demon-lords, arch-devils and other non true gods.
Temporals (Novice, 1st, 2nd, 3rd and 4th) This is where you get your demi god and lesser gods.
Celestials (Novice, 1-4) This is where you get the major and greater gods.
Empyreals (Novice, 1-4) (Ao?)
Eternal (Novice, 1-4) (the devs knows what from this point on...)
and you even have higher immortals
Hierarch (Novice 1-4)
And Full Hierarch.
Note: In the Immortal set, demons are fully immortal beings with PP. The above is just an approximation we did for D&D. It is by no means something official but the ranks are.
In terms of power, the lowest temporal has about 500 PP. Each PP can be used to cast or create spell like effect with 1 PP for a spell level or its equivalent. And this can be in addition to its own spell casting ability (if any). Imagine a 36th level wizards being a god... To show the difference, A Full Hierarch has 15,000. And they can create avatars over that...
In that expansion, immortals/gods can use PP to fuel spells, but it is also a currency with which they barter their actions. The more power you put in joepardy, the greater the gain, but the less you have for other challenges and personnal defense.
Example: In a game, a group of evil immortals came to the players and started a claim on a region where the players had set a religion going for them. The claim was control of the whole region by either the forces of evil or good. PP were placed has a bet and players opted to have a group of heroes to foil the plans of the evil immortals. It was the most cost effective. So they went back in time, got the mothers of our heroes to be met their respective husbands, use PP to make sure that the group would meet. Placed challenge (adventures modules, homebrew and whatever) on the path of the group and placed a few "key" magic items to be found by the heroes. Then their champion would raise to 14th level and fight the forces of darkness. And the way to do it, you guessed it, was to play this group heroes.
All this to show that there are interaction between the various mythos, gods and even aspiring gods such as demons and devils. Removing one of the other is fine. But the RP opportunities loss are greater than the gain in doing so IMHO.
