From a very recent signing, we have this new Word of Brandon...
chasmfriend's son: Is there a finite amount of Investiture?
chasmfriend's son: So is Nightblood consuming it?
Brandon: Yes. Very, very slowly.
This worries me somewhat because of the following observation.
Nightblood consumes Breath (and other Investiture, but let's limit ourselves to Breath for a second).
Every person on Nalthis is born with one Breath.
Populations tend to grow. Which means that under normal rules of demographics, population of Nalthis should keep increasing.
This in turn means that under normal circumstances the number of people with Breath on Nalthis should be growing.
I can see the following possible explanations to this:
Endowment can give Breath to many more people than are currently living on Nalthis. So, the exponential population growth has not yet reached the level at which Endowment's ability to award a Breath to each Nalthis-born human is seriously challenged. When it happens though, things will not go well.
There is some built-in mechanism controlling population growth on Nalthis, making certain that the population stays within the limits. Nightblood's consumption of Breath makes these limits smaller, and overall may lead to Endowment's inability to grant Breath to Nalthis-born, but not for a while (essentially, Endowment controls population trends at she sees fit).
Just as a point you should understand, the amount of MATTER in the cosmere is finite too. As is the amount of energy.
Worrying that Endowment will run out of Breaths to give is a little like worrying that the amount of carbon on Earth will run out because people keep being born.
So just for clarification, once Nightblood consumes investiture, that investiture gets recycled? That's what I've always assumed. That it enters the cognitive/spiritual realm?
The investiture he consumes is not gone forever--it's not leaving the system, so to speak.