It's not a matter of "LE and CE can't" do anything. It's a matter of "Demons and Devils have their own plots going on...and part of the hook of yugoloths is that they are interested in them both.
Law and Chaos are opposed metaphysical forces. Demons and devils have competing agendas. There's no reason to assume that these differences are meaningless. [\quote]
Law and chaos as opposed metaphysical forces is a Planescape invention. It does not occur in pre Planescape D&D.
Sure, that can happen. DL was big on the Good vs. Evil plotline, so it reduced the variation in Evil to provide a solid antagonist.
I wouldn't pretend that we can expect that this is always the way demons and devils in all D&D games would act, though. Devils can't rule a world the demons burn to the ground, and demons can't burn a world the devils lock away. Explicitly, the original yugoloths could be found helping along either team, implying that this is something that is worthy of note about them, ie: something that cannot be expected of just any creature.
Antagonists don't have to work together just 'cuz they all shoot red lasers.
Nothing I said should even imply that they "have" to work together. Just that the option is there.
You're being WAY to monolithic, here. It's not so binary a choice.
There's plenty of reasons demons and devils can't always be assumed to work together. You could have a scenario in which they did if you want -- nothing prohibits it. Even with the Blood War, you can do that (hell, one of the "yugoloth consipiracy" ideas is that they want to keep the Blood War going because it makes both demons and devils stronger for the upcoming War Against Good!). But equally nothing assumes it. No bit of the rulebooks says "Demons and devils always work together."
Going with an inclusive model of the game, you can say sometimes they grudgingly work together, and sometimes they break out into vicious all-out war. And then the DL fans run with one to its logical conclusion, and the PS fans who love the Blood War run to the other. At any given table, you may ping anywhere on the continuum.
And, it's a good thing I never said that they always work together isn't it?
But, post Blood War, you're flat out stating that they can't.
You stated,
There was. It was called "alignment."
LE and CE don't get along. And because they're both E, they really don't get along (central to the idea of evil is advancement of yourself at cost of others, after all).
which is not really hard wired into alignment. Sure, they don't get along. They're evil. But, that doesn't preclude them working together. Post Blood War, they can't. You've stated that multiple times in this thread. The difference for Yugoloths, according to you, is that they can work for either "side". Pre-Bloodwar, there isn't a "side" to work for. All the demons and devils are not locked into this monolithic struggle.
So, if demons and devils can work together, then why can't I have a vrock and an erinyes working together? Why do I need mercenary Yugoloths?