Ah, I didn't realize you had put agreed with the sentiment that there is no real purpose to the Evil Gods, beyond having them to have them. Yes, if you have reached that point then the only thing left is to homebrew solutions, and I liked some of yours in theory. I had my own that I implemented.
I meant more that there aren't any canon reasons why gods and arch-things aren't after each other's jobs all the time. But anyway. I'm not 100% convinced there's any real purpose for
any gods (or, any gods that are verifiable by mortals, at least). This thread has encouraged me to come up with no-gods universe. Which will get put with all of the other settings I've come up with that I will probably never use.
Evil gods exist in real-world mythology, even if purely evil gods are rare. For example, I'm no expert on Finnish legends, but their Loviatar seems to be about as evil than her Faerun counterpart--but a lot
cooler, judging by the poem written. So while there's no real reason to have evil gods, I don't think that there's a reason to say that one shouldn't include them--or that they are interchangeable with arch-fiends.
Just checking, because the end of the Blood War is generally considered to be the point when the winning forces turn to the Material plane, so I'm not seeing much of a difference here.
I'm pretty sure the Blood War is never going to actually end (unless there was some adventure or sourcebook that said otherwise, in which case: that's stupid; the Blood War is never going to end).
But anyway, that has nothing to do with the war between Maglubiyet and Gruumsh, who are fighting because... I dunno. Probably for the same reason the actual Blood War exists--to explain why these two different people haven't teamed up and taken over the world yet.
An interesting way to take it. Except that the Devils do make deals for souls. Sure, maybe any given Lawful Evil soul gets taken to the Nine Hells, but then it is just picked up by whoever. Devils who don't focus on the material plane to some extent tend to end up weaker, unless they have something to offer those hags, because they have to fight and search for souls, while those who make contracts are guaranteed souls.
The 2e Planescale MC says that that night hags are always willing to trade for knowledge and magic, and for certain favors: "lower planar powers ... agree to not enter night hag territories" and "liches destroy creatures who refuse to trade with hags" in exchange for larvae." I imagine there's plenty of other things that can be traded as well--services, slaves, and so forth.
Also, it doesn't seem like night hags have to search all that hard for larva. They're listed as having the frequency
Common and the organization
Masses, and the flavor text for the entry had some adventurer encounter a night hag with a herd of several thousand larvae. (The 2e entries for both night hags and larva have tons of plot hooks. I miss the 2e monster writeups.)
We also don't know the exchange rate. For all we know, a
potion of healing might be worth one larva. Or a dozen larvae. Healing might be hard to come by in the lower planes.
I'd say the same thing happens with the Demon Lords. Worship them, pledge to them, and your soul is send directly to them, instead of them having to grub in the dirt for a larva. It isn't icing, it is delivery.
The 2e book suggests that larvae only show up on the Gray Waste. Since the only major fiends that live there are the night hags, neither demons nor devils can gather larvae on their own without going to another plane and invading night hag territory. I can't find info on them for 3e (it probably exists; I just can't find it), a small entry I found online for 4e shunts "soul-larvae" to the Shadowfell (which I'm guessing replaced Hades as the place for evil souls to go), and their 5e statblock is missing... everything useful or interesting about them. Sigh.
The 2e MC says that larvae are the only way to make imps and quasits (and that imps and quasits can then be turned into higher-ranking fiends). But 5e says that
lemures can be turned into imps, and lemures don't start as larvae; they're also created directly out of souls. I'm guessing maybe manes can be turned into quasits? So larvae are back to being food (4e apparently gave benefits to creatures who ate a larva, because nothing's just
food anymore), a fuel source, and a currency.
So, back to souls. Have you read or watched Good Omens? Specifically, the bit at the beginning of the story (but not the
very beginning), where the demons Crowley, Hastur, and Ligur are recounting the Deeds of the Day. It's a 30+ year old book, and this particular scene is more about establishing character than plot, so I'm too lazy to spoiler it. Hastur and Ligur have spent a lot of time tempting two people, thus securing those two souls for Hell.
Crowley tied up all the mobile networks in central London for half an hour during lunchtime, thus ensuring that millions of people became frustrated, meaning a lot of those people will take out their anger on others, who will take out
their anger on still more people--meaning millions of people got a little bit more evil and, therefore, a little bit closer to Hell.
So that's the problem with signing souls. If you need a lot of souls (one of the plot hooks in the 2e MC was that the baatezu were making a doomsday device and needed
millions of larvae to power it), then individual contracts aren't going to cut it. Getting individuals to sign away their souls is, as Crowley put it, craftsmanship, but this is what I mean by it's icing: it's honestly not enough for any major purposes. It ensures that the soul goes directly to the signer. But what's
one soul? Well, in D&D, that soul would be probably start out as a higher-ranked fiend, which is definitely useful for a variety of reasons--but it won't be useful for food, fuel, or currency.
This is why there are cults, because it's extremely likely that the cultist's soul goes directly to the fiend it worshiped. But it's not totally guaranteed that this will happen, because redemption
is a thing--and in at least some earlier editions, there was the
atonement spell, which I imagine could be used to cleanse a soul.
Now, it's possible that an arch-fiend would also gain power from being worshiped--I think someone else posted a quote that suggests Asmodeus does--but I'd say that indicates that the arch-fiend in question is either actually on the cusp of godhood or
anything can gain power from being worshiped. D&D only
barely touches that latter concept, probably because it would be difficult or at least page-consuming to come up with actual rules for it.
Perhaps instead of being a duality, the difference between gods and arch-things is more of a continuum. The more god-like you are, the more power you gain from worship and the more you can create and control, and the more arch-thing you are, the less dependent you are on mortals (going with the idea that gods die without worship but arch-things don't) and the more you can
interact with the Material World without damaging or altering it. There's benefits to both sides.
Also, while it may have fallen out of style, there is the divide of the mortal into soul and animus, which means that they might be able to get the soul for power, and use the animus to create a fiend, making it a two for one deal.
That would be cool. I do wish that things like this were given more time in D&D. At the very least, a fiend should be able to split a soul into two (perhaps using eldritch machinery, because eldritch machinery is always fun), even if the soul was originally a single thing.
I disagree entirely. The Material plane is the only source for more devils, you may have them showing up randomly in the Styx, but that is much less efficient then getting them to pledge directly to you, denying all your rivals the potential to obtain those mortals.
Demons might care less.... except that they are embodiments of destruction, they have an intense desire to destroy the material plane, by their very nature. So, there isn't really a reason for them not to go to the prime, at least as much as they currently do. After all, they don't just need resources to fight Demons vs Devils, but also Demons vs Demons and Devils vs Devils... conflicts that are still ongoing despite the Blood War
The Material Plane is finite. Or rather, even if space is infinite, each world is a finite area, and outside of
really long-range teleportation and Spelljammers each world is isolated. The Hells and the Abyss have truly infinite living area. Demons are destructive, but they're also like a cancer: they destroy by growing and choking out all the healthy life--but when the life is gone, that means that the cancer is also destroyed (although with demons, it's because they would turn on each other if there were no non-demons to play with). Demons will eventually truly destroy a planet and take themselves with it. Now, for the average demon, this is no biggie. They can't think
that long-term. But the demon lords are smart enough to know better. They know how to pace themselves. They're not going to send entire armies through to the Material just for funsies.
Also, doing so would attract unwanted attention from other fiends (who might interfere, if only to keep the invaders from becoming too powerful), good celestials (who need to protect mortals), and the gods (who want their worshipers unharmed), as well as mortal heroes.
Edit:
And since we have shown quite a lot of evidence that they aren't the only source of clerics, that even the rules that state Demon Lords and Archdevils can't grant spells are caveated with "unless you want them to", and that they aren't necessarily more powerful... the conclusion seems to be that they are fairly redundant with Archfiends. And therefore their role can be handled just as easily by Archfiends.
D&D is full of redundancies. Ogres are redundant with hill giants and verbeegs, which I can't believe they brought back. Orcs are redundant with goblins who are redundant with kobolds who are redundant with xvarts. Orogs and ogrillons are redundant with each other. Dragons are redundant with each other. Giants are redundant with each other. Pixies are redundant with sprites.
D&D is full of redundancies. It's a messy, complex system. That's part of its charm.