How is the Blood War a balanced conflict?

Question

There are 2 sides to the Blood War. The Nine Hells and the Abyss.

Devils, from the Nine Hells:

• Are Lawful, orderly and disciplined
• Take the souls of mortals they can make deals with either in life or after death, and these souls are their only source of new devils.
• Their variants listed in the Monster Manual are on par with those of demons - i.e. neither side seems to have overall stronger beings in their ranks
• Can only permanently die in the Nine Hells

Demons, from the Abyss

• Are Chaotic and unpredictable
• Take the souls of some mortals they can steal from the Wall of the Faithless to make minor demons
• The Abyss has many/infinite layers, and on each one new demons are birthed.
• Can only permanently die in the Abyss

Given this, for the conflict as a whole:

• The conflict is said to rage on the upper layers of both the Hells and the Abyss, so sometimes demons will be permanently killed, sometimes devils.
• In a conflict of equal numbers, devils will probably come out on top due to greater discipline
• However the Abyss is constantly generating new demons in what must surely be much greater numbers than devils can tempt mortals.

Why then do canonical sources suggest that this conflict is roughly balanced? i.e. sometimes the demons get the upper hand, sometimes the devils, and Mordenkainen and his "Balance" lot put their fingers on both sides of the scale to make sure neither side dominates?

It seems to me that there must be some factor that I'm missing in favor of the devils, to balance out the huge numbers of demons spawned in the Abyss, which would otherwise just lead to the numbers of the Hells being whittled down to nothing in a war of attrition - and that the greater discipline of the devils is not a sufficient factor to balance it.

Answers should ideally be taken from official 5e materials or designer statements, but if these are insufficient for a full answer, material from earlier editions could also be used.

Answer 1

This is addressed directly in Mordenkainen's Tome of Foes

The first chapter is all about the Blood War. A very simplified and brief synopsis of the reasons the Blood War is a stalemate:

• Demons have quantity
• Devils have quality
• There are groups and individuals acting as intentionally balancing forces who are sufficiently powerful to manage any side that might get the upper hand.

The Balance

On page 8, there is a description of the sorts of players who intentionally keep the balance where it is in the Blood War.

... A third point of view exists, held by those who take both sides at their word and strive to make sure that neither outcome ever comes to pass.

The adherents of this viewpoint ... seek to maintain equilibrium across the cosmos above all. Mordenkainen and his compatriots are among its most notable devotees... Those few, however, make up a formidable force that can mix martial and magical power to keep the extremists of the Blood War in check.

General Statemate Summarized: Progress for both sides has difficulty

Demons have difficulty controlling a horde

(MtoF p.7)

More important, demons that are slain and sent back to the Abyss return to their chaotic wanderings in that realm. A demon lord is thus hard pressed to keep a horde cohesive as it takes casualties.

Devils have their own material concerns

(MtoF p.7)

If the devils extended a tenacious defense out to the planes beyond Avernus, they could keep the demons away from Hell's doorstep, but such a strategy would place a great strain on supplies, reinforcements, and unit integrity. Although the devils killed in such places would recover, their weapons, armor, and other materials would remain lost.

Minor points about permadeath

Lemures

The foot soldiers on the devil's side don't suffer permadeath in the same way devils and lesser devils do. (MToF p. 7, 17)

Hordes of lemures, devils that are permanently destroyed only if subjected to holy energies, are used to blunt demonic incursions.

Devils may care

The prospect of being destroyed permanently is not seen as a detriment to the devils. (MToF p. 7)

Also, the prospect of a permanent death compels the devils fighting on Avernus to maintain the utmost readiness and coordination. Wounded devils retreat, knowing that fresh reinforcements can continue the fight.

Mercenaries

There are a class of fiends that don't get destroyed in either of the major arenas of the Blood War, and thus are a constant source of martial power for both sides. (MToF p. 8)

Yugoloths are major players on both sides of the Blood War. The death of a yugoloth is meaningless when it occurs in the Abyss or the Nine Hells, and thus these mercenaries freely give their all in any battle even when it means fighting other yugoloths.

Answer 2

No one in power wants the Blood War to end just yet

Neither Hell’s archdukes nor the demon princes are especially interested in ending the Blood War right now. Both have their own schemes independent of the Blood War that they would rather pursue than the war itself. All of them do want to win the War, and assume they eventually will, but none of them are particularly set on that happening within any particular timeframe.

Hell does take the Blood War more seriously overall; a whole lot of the Baatezu hierarchy is devoted to it. The Dark Eight, the pit lord generals in charge of the war, are second only to the archdukes in the hierarchy—and one of their number, Bel, has become an archduke.

The Abyss, on the other hand, is full of demons who generally don’t want to do anything anyone else tells them to do, and only some of them personally want to go to War. There are some demons who press-gang other demons into fighting in the Blood War, but that often literally requires holding a knife to their throats and forcing them to do it, so there are stark limits on how much influence any one demon can have—and again, the demon princes have near-zero interest.

Beyond Hell and the Abyss, absolutely everyone else has a vested interest in the Blood War continuing. The yugoloths maintain that the entire War is their own scheme, that they manipulate for their own purposes and that the War will end only when they decide they want it to. Mechanus doesn’t often take a role in the War, but when they do it is on the side of Hell. Slaad are less helpful to the Abyss, obviously. And the Upper Planes have been known to interfere in the Blood War if it looks like one side or the other is getting too much of an upper hand. Because basically no one wants to see the might of Hell or the Abyss unchecked by the other. The end of the Blood War will mean that the victor turns their sights on the heavens, and they’re probably going to go through the planes in the middle to get there.

So the Blood War continues because the forces invested in perpetuating it are vast, without any countering force looking for a decisive end to the War any time soon. The balance in the Blood War is to everyone’s liking.