system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40,000] During the Dark Age of Technology days, if the Emperor had used all of his behind-the-scenes manipulations to steer mankind towards building and colonizing a Dyson Sphere around our Sun, then how would 20k, 30k, and 40k history change?

Before the great scattering that would see mankind spread haphazardly throughout the galaxy, the Emperor of Mankind uses his skills and resources to change the human sentiment just enough to have humanity decide to stay within sol. 

Instead of spreading out and colonizing the unknown, we instead use our vast DAoT tech and resources to simply build millions of O'Neil Cylinders and Mckendree Cylinders around our sun instead, thus forming our first Dyson Sphere.

Now, instead of being placed chaotically all throughout the massive void of a very diverse and dangerous Galaxy, **we are instead** all bunched up in our home system, albeit distributed reasonably well accross literally millions of cozy, heavily armoured (and armed?) solar-powered habitats.   
*For the sake of argument , let's assume that the raw materials for all of these habitats was gleaned from planets and asteroids in Sol's sorrounding systems/sectors. Also assume that the majority of humanity is inhabiting the Dyson Sphere, with only a relative few separatists, wealthy elites, or researchers staying on Earth/Mars or privately striking out into the wider galaxy on their own.*  


Now I ask you, **how does this preference for the Dyson Sphere change humanity's condition and fate during the 20k years, the 30k years, and of course, the 40k years?** Do major human trials like the Age of Strife, Great Crusade, Horus Heresy, War of the Beast, and so on all go down the same way, and if so, are they less or more destructive than canon?

Why does canon DAoT humanity neglect to build their own Dyson Sphere in-canon, anyway?
gpt
So what if humanity never left the solar system over the entire course of the multiple tens of thousands of years of their civilization?

Hmm...first problem is over-population. Exponential growth over that time scale is nothing to be scoffed at. With everyone trapped in a single solar system, that’s a recipe for conditions getting very bad, very fast. 

But let’s say for the sake of argument that some way is found around that — the problem might not be entirely unsolvable. 

The Men of Iron revolt still happens. There’s nothing in the scenario that prevents that. If the same level of physics-breaking superweapons are deployed, that might be enough to end humanity right there, or at least do so much damage to the sphere as to render it mostly non-functional. 

But even if it somehow doesn’t, Slaanesh is still birthed into the galaxy soon after, and the resulting warp-fuckery that is the Long Night still happens. Warp storms aren’t the only result of that — the records of the Unification Wars of Terra record all manner of psychic abominations that date from that time, the implication being that they too, may have been an indirect result of the opening of the Eye of Terror. It is at least true that Terra was crawling with violent, potentially Warp-tainted barbarians, monsters, and psykers, which even the Emperor had to spend quite a bit of effort to pacify. *That’s* what your humanity now has to deal with on their Sphere, only even more so because of how much bigger it is. 

Not a great outlook. 

One of the few things in the 40K canon that’s relatively consistent between portrayals is that the only reason that the Imperium has survived this long is through *sheer goddamn inertia* — it’s *too big* and humanity is just too widespread, that even after 10,000 years of being attacked on all sides by all manner of aliens and daemons, with countless lives being lost every single day, it *still* controls a major portion of the galaxy. It’s not *enduring* so much as *not dying fast enough*.

Restricting humanity to a single system would only seem to make it more vulnerable, not less. 

Put it this way — if “building a Dyson Sphere” was an easy answer to the problems of the 40K universe, then neither the Necron nor the Eldar, who routinely move, capture, and play around with *stars*, should ever have had any sort of problem.