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.