Nyrrix

I would postulate that the Father Machine may be working as an Investiture pump, on some level. We already know from The Stormlight Archive (especially according to Navani's book 4 chapters) that Investiture can have differentials similar to pressure systems. In this case, Father Machine may have needed to be primed in some way. But without a set of hijo already available, it pushed human spirits to start the process. While they were going through (and creating the waste in Investiture in the form of the Shroud), Father Machine could start stacking and attracting hijo. After it got those first hijo, it began to fulfill its purpose in true and began the process of making hion. It stopped using human souls (but kept the yoki-hijo imprisoned because they were classified as a threat to its Intent). The pump could be perpetuated since the stacked stones created a "positive" pressure on the input side and the hion a "negative" pressure on the outside (sorry if I mix up terms for pressures, I'm still an undergrad). Painter's people used hion, so the differential was maintained, and the Father Machine probably needed the differential to run (similar to how a dam's computer systems and lighting might be powered by its own turbines). This brings up other weird questions, like "Why do human souls produce the Shroud and not hion?" And if these are pressure differentials, "Why didn't Father Machine stop working immediately after society fell, since the hion couldn't be used (presumably) immediately after by humans? (The pressures would have equalized and the spirits probably dispersed when it stopped stacking)." That's just a theory. But I find it compelling. It falls somewhat in line with how Spren are captured in gems (by removing some investure from a sphere). This could be that on a larger scale. Investure extiting and hijo entering in response If this is the case, it's a different function that seems like it would need less investure to get going and less investiture for the process to be maintained. As others have said, Nalthian awakening does a lot more than what Father Machine seems to do. Nalthian Awakening is also a (comparatively) static system, so it needs to be stable. Ergo, probably needs more Investiture (as seen by Night blood being the most invested entity in the Cosmere, barring shards and some splinters and avatars).

I was noticing a bit of a parallel between Nightblood's smoke and the Shroud. I wonder if the smoke that falls off of Nightblood is a similar to the waste substance (the Shroudn) produced by the machine as a byproduct in Hion synthesis. The substances both look like smoke and seem to defy natural laws/gravity (Nightblood's smoke falls, the Shroud seems to roil and sit on the planet). Both (the machine and Nightblood) are also manufactured Invested fabrials/devices/entities. They also have an Intent, but that Intent was ill-defined by their creators. There could possibly be larger-scale indication for how Investing objects works (and how it fails) in the Cosmere.

Going beyond the meta-value apocalypses have in story telling: Earth has seen some rather devastating apocalyptic events. They completely changed the way humans interact with each other and the world. To name a few: World War II and I (global relations were flipped almost completely), the black death, the Bronze Age Collapse, and the Great Flood (I mean the end of the last ice age, which separated Russia and Alaska, flooded a lot of coastal regions, and seemed to inspire a universal flood myth in a lot of ancient cultures). I think there's a tendency to take our own apocalypses somewhat for granted, but notice a fictional property having enormous, far-reaching events and say, "That's not realistic!" Sure, some of the apocalypses Brandon has cooked up are on a ridiculous scale, but many are not unreasonable if you set aside the magical context. I don't think these world-changing events would not be so enthralling if we didn't have quite a few shaking up our own world in the past.