The thorny part of this discussion is that there have been at least three distinct “eras” where certain things were true. Yes, in the earliest days when these beings were first created, the clerics of Demon Lords and Archdevils were limited in their ability to cast higher level spells. But, at least by 3.5 according to the Fiendish Codex’s I quoted, that was no longer the case. They were explicitly allowed high level spell access. Now, in 5e, there is some confusion on the issue. Because a cleric of Asmodeus or Orcus wouldn’t be limited in spells, but it maybe argued that they are drawing power from a philosophy, not the being itself,
I'd say that perhaps clerics of an arch fiend can have a limited number of
clerics. They have only so much power to lend out. They probably can have a lot more
warlocks, because it can be argued that most of a warlock's powers come from the patron zapping spellcasting knowledge inside the warlock's head and making the warlock draw on their own inner magic to cast the spells (hence the small number of slots, but their "mana" replenishes quickly), whereas a cleric or other divine caster actually gets both the knowledge
and the mana from the god/philosophy/earth itself.
So a god can have absolute boatloads of clerics, but an arch-thing can have only a tiny fraction of that number. They only give clerical powers to people they know are in it for the long run. It's much cheaper for them to make warlocks, even if warlocks have a reputation as being, well, warlocks instead of as clerics.
That's how I'd do it, anyway. I'm not huge on the idea of people getting high-level spells from a philosophy that they don't even recognize or accept as a philosophy. I think that by saying you're getting spells from a god, you're unnecessarily limiting your own abilities. You actually believe strongly in a real philosophy (even if it's one of your own making), then sure, you can have full spellcasting. But people who are convinced the magic comes from elsewhere, no. Also, then a lot more people should have spellcasting, since philosophy doesn't just mean philosophy in this case; it means strong belief, and people can believe very strongly in a whole lot of things.
One reason I haven’t been discussing Exandria too much is because I’m uncertain if they actually have Demon Lords. I know that Asmodeus is a god there, and I know Orcus existed in the campaign, but I don’t know if Orcus was also a god or how he was considered in relation.
There's a whole bunch of "lesser idols" (According to the Explorer's Guide to Wildmount; I don't watch the show).
"This vacuum of influence [when the gods were banished] has given rise to a number of powerful entities who may not rival the gods in their abilities or influence, but now unchallenged, can amass a modest following of their own." They have both suggested warlock pacts
and cleric domains, although I think the idea is that they have more warlocks than clerics?
I can't find anything on Orcus in that book, but it covers just one part of Exandria and as I said, I don't watch Critical Role or really know anything about it beyond what's in this book. And it's been a while since I've read the book since it's not the most interesting of reads for me. So I have no idea how he's treated there.
So, how do we justify “the gods are more powerful” in this dynamic? That’s my question.
There's another questions here: Bane is recognized as a god on a couple of worlds. And probably there's some planehopping sages on Oerth or Eberron or Krynn or in the middle of Wildspace who have a copy of
Elminster's [Dating] Guide to Gods who would say "So there's this god named Bane, god of war, tyranny, that sort of stuff. He's not worshiped on
our world, though;
our world doesn't have gods with names like comic book villains. What were those Faerunians
smoking?" But on these worlds where Asmodeus is known about, is he known as a god or as an arch-thing?
In other words, if Gods Need Prayer Badly, then perhaps the reason why Asmodeus and other such entities as arch-things instead of gods is because most people think that Asmodeus is an arch-thing and not a god.
(This is why D&D needs to publish Immortals rules. The heck with epic levels; I want rules for godhood.)
If Asmodeus is a fellow god, then how can Bane easily kill him? We have yet to see a world where they are not both considered gods, according to previous discussions. And how is Asmodeus retaining control and influence in countless realms if he is weaker than a god who can’t even conquer a continent?
Where Asmodeus is a god, he's sometimes the god of tyranny and sometimes the god of indulgence and sin. He's effectively multi-classed. Bane's war and conquest. Even in Ravenloft, he only had law and order
added to that; he didn't get a different portfolio. Bane is
specced for battle.
I mean, I know that this sounds silly, but in a way it's not. Bane's a greater god, but so is Jazirian, who is the couatl deity of peace, community, and parenthood. And I'm sure you have no problem imagining Bane stomping Jazirian's feathery tail. Bane might not come out of such a battle unscathed, but he's not going to lose. So just because they're of equal rank doesn't mean that they're equals on the battlefield.
Also, Bane isn't actually dropping down to Faerun and murdering everyone himself (unless he actually has; I don't know from the Realms). He has his clergy and other minions to do it. And
his minions are countered by the minions of other gods, many of whom are willing to work together to prevent Bane from taking over the world. And I'm fairly sure that open worship of Bane is frowned upon in the Realms, so even though he's powerful he doesn't have a lot of overt support.
Whereas Asmodeus can't count of getting help from other arch-things, and how many of his warlocks just grabbed power and then headed off to do their own thing? Warlocks aren't beholden to their patrons for power in the same way that clerics are beholden to their gods. And Asmodeus has relatively few clergy--who, like with Bane's clergy, are also being thwarted by the minions of other gods.
And on the notion of Pride, what about Asmodeus’ pride? He is the unquestioned ruler of the Nine Hells, why would he allow some god who can barely reach past his homeworld to challenge him?
Because it might not be worth it for Asmodeus to do anything. If he kills Bane right now, what does he get? A helping of Bane's portfolio? (The Realms' god-portfolios seem to work like Highlanders) And what does Asmodeus actually want to be? A god of tyranny or a god of sin? If he wants to be a god of sin, then killing Bane does nothing to further that goal. If he wants to be a god of tyranny, then he has tons of other worlds to conquer first. If Asmodeus is actually powerful enough to kill Bane--which as I pointed out, he might not be--then there's still not enough of a
reason for him to do so.
There's honestly not enough reason for either of them to care about each other. There's cults of Asmodeus in the Realms, but they're not messing with Bane. There's churches to Asmodeus on various worlds, but Bane isn't messing with
them. But as I mentioned before, the On Hallowed Ground book considers Bane and other FR gods to be weaker "single-sphere powers." Asmodeus is actually probably more threatened by Tiamat, since she's at least a bit interested in world domination (or at least, in the supremacy of evil dragons, which is kind of the same thing),
and she's a multi-sphere power
twice over--once for dragons, and once for any world that worships the Babylonian gods--
and she's a generally-acknowledged god in both Exandria and Krynn.
And there's one other reason for them not to fight as well--they could potentially
ally with each other. Sure, I can see Asmodeus (since he has levels in God of Sin) actually end up betraying Bane in the end, but I can imagine them teaming up to take over a particularly difficult plane/sphere.