Sure, I'm not against any of this.
Though, I would say, that if we are considering Archfiends to be in a near infinite number of planes, even if they only have one or two clerics per plane, they still can create far more than a god can.
What is bizarre about how this discussion is starting to turn is that people started from the premise that the gods are stronger, but it seems that is continually being proven not true. The Fiends are simply stretched thinner, making the gods appear stronger, which then removes the impetus to have evil gods, because it seems they aren't actually doing anything that unique.
Being stretched thin
does make Asmodeus weaker. Even if there are literally ten million Asmodean clerics stretched across a million Material Planes, most can't communicate with each other across planar divides. They have no ability to work together to perform any great deeds, and may even end up working at cross-purposes at times. Whereas there may be only a few hundred Banite clerics in Faerun, but since they were a very organized group in Ravenloft, I imagine that's also the case in the Realms as well (the faith hasn't taken over Ravenloft because it's under a curse where it will never be able to gain a foothold in any domain other than Hazlan and Nova Vaasa).
Now, is Asmodeus actually gathered a large chunk of spellcasting clerics and warlocks in one area, they'd definitely beat Bane's clerics--but he hasn't done that so far.
But
anyway, all of this is besides the point. There's no particular reason to have evil gods, but then again, there's no particular reason to
not have them either. And no reason why you can't have both evil gods
and arch-fiends. Arch-fiends could be the generals of the evil gods. Or they could have their own little Blood War and are keeping each other so busy fighting amongst themselves that they aren't constantly attacking the Material world.
However, I think there's one thing you're forgetting when you said "that if we are considering Archfiends to be in a near infinite number of planes" which is:
none of those planes exist unless the DM wants them to. I, personally, don't have that sort of multiverse in any of my homebrew settings, and it plays only the most minor of parts in my Ravenloft games (and only then in the sense of, if you're playing an Outlander, you came from "another world."). Asmodeus has no presence on millions or billions of worlds because there aren't millions or billions of worlds. If I were to run a Planescape or Spelljammer game, that might change, but I haven't run one of those yet, and I probably won't. And if I
did run something Planescape like, it would probably be one world and it's associated planes.
And I have a sneaking suspicion that a majority of DMs are like that. I'd guess that actually relatively few DMs really care about the multiverse, especially in the sense of how it powers the various gods.
Which, rolls us right back around to "what is the practical difference?"
Well, what would you like it to be?
In a much earlier post ITT, I posited that gods can actually create a permanent thing out of nothing. How much a god can create on its own probably depends on its power level and portfolio, but it's something. Arch-things, no matter how powerful they are,
can't create
ex nihilo. They can transform something that exists into something else (again, probably depending on its power level and portfolio), but they have to have an existing thing first.
So that's my practical difference.
Possibly, so then the question is, does Godhood become just a title, while the essence and power of the title holder does not change? Or is this something else?
Or you could go with this. That being a god means you finally got accepted into the god club, and arch-things are just too déclassé to be allowed in. Not being in the god club means any mortal who worships you is going to be seen as slumming it. There's probably some arch-things who have wheedled their way in by being PAs or arm-candy for some of the gods.
Yeah, OK, this is
also silly, but it's still plausible. I mean, they probably wouldn't use that sort of terminology, but who knows?
See, I don't disagree with this. I do agree that Jazirian likely loses a head on fight, with how DnD handles things. But does that make him less powerful? And, Asmodeus is also very very good in a fight. Just for an example, Zargon the Returner is a GOO that the gods couldn't defeat, so Asmodeus came to the Prime and beat him and sealed him.
Physically, yes, Jazirian would be weaker than Bane. The two gods may have equal powers, but they're expressed differently. Now, if Jazirian was given the opportunity to, I dunno, create a field of peace and love around it, then Bane would at have at least a chance to succumb to it and end up being too mellow to fight.
So in reading up on Zargon of the FR Wiki. One, Zargon is a GOO, and it's safe to say that they work differently than gods or arch-fiends. Two, it appears that Zargon had "god-disrupting powers" and Asmodeus was immune to them because it's implied that he wasn't a god at the time. Three, Zargon had
just been in a multi-day battle with a barbarian named Zenkar until Asmodeus took over the battle, which probably tired him out a bit. Four, Asmodeus killed off large amounts of Zargon's followers ahead of time, and offed most the rest while sealing it into stone, so if gods do need prayer badly, Zargon is SOL.
So basically, Asmodeus is a kill-stealer who managed to fit in the "no gods" loophole. It's kind of a "no man born of woman" thing. It's not really about power here.
As I mentioned, the idea of "sin" in DnD is a bit nonsensical to me. And sure, they could be allied, with both planning on betraying the other, but I think the other idea highlights the issue.
If one of the logical reasons Asmodeus hasn't killed Bane is because he is too busy with other threats and other conquests, how can we say that Bane is obviously the stronger? Remember, then question we have been trying to tackle is "what is the role of evil gods"? And the most common answers have been "evil gods are stronger" and "evil gods make clerics". If Asmodeus can make clerics, and can be logically considered powerful enough that he just hasn't bothered to kill a god... then neither of those points actually support the existence of Evil Gods.
Well again, you would have to figure out what godhood actually
means. So far, the 5e books don't say, so you have to make stuff up. It could be the creation thing I came up with; it could be that a god can make ten clerics for every one an arch-thing can make; it could be there's no practical difference.
It could even be that "god" is a
species: you are born a god, or a god passes its divine DNA onto a mortal being via spiritual horizontal transfer when it dies, or maybe you're actually the descendant of a god and a mortal and your god-genes activated--but unless you have those god-genes, you're not a god. It's then just a question if arch-things can engage in god-diablerie.
The problem with saying that if Asmodeus is powerful enough to kill Bane, that it doesn't support having evil gods in a setting--well, then why have good or neutral gods either? Why not send Asmodeus after Tymora, or, I dunno, Psilofyr?
See, this gets into another question: where do gods come from? Did they come before mortals or did mortals dream them up? (If they came before mortals, then they probably can't need prayer to survive, because they would have died out before they invented mortals. Unless there used to be ambrosia a-plenty but it's all gone now.) Did they always have the same portfolios, or any portfolios at all, or did mortals decide that Bane was going to be the god of war and conquest? Was he always evil, or did he start out OK until mortals decided that war and conquest were generally evil things?
Which came first, the god or the alignment?
I ask this because, as I also said earlier ITT, four of the six the orc gods don't have evil portfolios, but they're listed as evil because, well, orcs are Always Evil, so of course they would have Always Evil gods. They have an Evil god of Evil strength and loyalty, and an Evil god of Evil hearth and home.
Which is silly--sillier than a the god club. It's The Sims silly, where an Evil character eats Evil breakfast and takes Evil showers.
So, if you're questioning as to why there needs to be evil gods in a setting, figure out whether those gods actually need to be evil in the first place. Bane is war and conquest, but maybe he isn't conquering poor innocents and bringing forth a tyranny of brutal misery. I'm no expert on Hinduism, but according to Wikipedia, the goddess Kali "destroys the evil in order to protect the innocent." What if you change Bane to be similar. His purpose could be to destroy corrupt and evil nations and groups. You get some leeway of "what counts as a corrupt," which I'm sure some followers will take to mean "anyone I don't like," but it doesn't
have to be like that.