In my previous question about galactic empires the subject of human infantry was not the main focus so it was not discussed properly.
And so I think it needs to be covered as I'm really trying to figure out if the military of the future would still use human based infantry with this particular world in mind?:
- The scale of the world is something like Warhammer 40K without magic or insane ideologies. I just mentioned the scale because in Warhammer 40K titans are a thing and in the past there were something close to aircraft carriers. So the scale is galactic - tens of thousands of populated worlds.
- The cost of making a fighting robot that is much better than a human at war is nothing more than few hours of machine work. This is how everything gets done anyway from the simplest things to the most complicated. Humans barely supervise anything and all is done by AMI.
- There is no true artificial intelligence. Instead it is AMI (Advanced Machine Intelligence) and it is basically AI-like but not human levels yet. It is capable of making human level tactical decisions and has access to vast databases and can be deployed on computational substrate the size of a fist. A machine intelligence does not prioritize its own existence unless programmed to do so; that said, it is obviously made to save energy and the machine it is in. Its cognitive performance across the board is similar to humans. AMI units run most aspects of society.
- The social situation of the empire is that all humans are on welfare. So going to the military for money is meaningless.
- Fighting machines are 100% proven to be better than humans in every military context.
- Transporting, cost or efficiency again make using humans seem very stupid. For example, let's take the excellent orb suggestion from the most voted on answer. Now the orbs would be huge in size, say 15 meters, but they can be very easily shoved in a storage space without worrying about even air for them. And that alone saves money.
- Humans then would be officers on the level of controlling a specific theater of operations. So a small group of officers would command the military operation from the ground so to be under the protection dome of the machines which is like a several kilometer area of protection from enemy hacking or jamming...etc.
- Whatever anti technology an enemy can come up with it makes sense then to come up with a solution for instead of trying to throw people in the meat grinder.
- The expected galactic battlefield is so unpredictable that humans would need insane training and equipment to expect to survive. So assume a very high gravity world and you have to give humans expensive special armors or a world of ammonia atmosphere and you also have to worry about a single rupture in the armor while machines don't care...etc
- The cost of bioengineering people or cybernetically enhancing them and so on is still more expensive than just making war machines.
- Like I said before, space battles are the primary theatre of combat. But one concept I'm still adamant about is that humans will need fighting machines to arrive on a planetary surface to do things, whatever those things are. So while it is true that the focus of all galactic powers is on capitol ships and planet killer weapons, ground warfare still exists.
- This is in regards to the standard practice of the galactic wide defense strategy of humans. Desperate situations don't count. As we all know it is theoretical to turn every person capable of using a rifle in a country into a soldier, but that is apocalypse level thinking and not standard theory or practice.
- I love Warhammer 40K to death. But they do their own things to justify infantry, and honestly I think they work.
- Mass Effect, for example, is nothing like the huge scale of my universe, so I don't think it counts.
I can provide more information; this is all that I think is relevant now.
Personally I thought of having some people still choose to be infantry, but as either officers wanting to gain more experience or insane zealots that the military sees fit to throw away in the nearest conflict.
Knightly organizations, thrill seekers, etc. are not part of the military. As a result, they are not discussed here.
I'm providing these aspects not to limit or force an answer but because they are how the world is set up. However, I have no problem with an reasonable answer that proves that infantry is still useful or a part of my concept is conflicting or anything.
So. The main question remains: In this universe would humanity use infantry? Why or why not?
Edit. Since AlexP asked for clarification on the scale of Warhammer 40K and my world: Humans control vast parts of the universe and thousands of stars. That's the closest approximation; the actual number isn't relevant.