Sure the architecture of Nexus looks old, to some more than 100 years old, but I have to ask - why do people think the Kantus had anything to do with that? As far as I can tell, the concept of the Kantus building Nexus originated with Tao Devil's pre-Gears of War 3 Locust origin thread, viewable here, which also seems to be the origin of the "Myrrah is the only successful experiment from New Hope" idea that I keep seeing Locust Forever and others mention. This theory was posted on 02-10-2010, before Anvil Gate, Coalition's End and the Slab were published, long before Gears of War 3 came out, and during the long comics hiatus between Barren parts 3 and 4. At that time all that was known about the Lambent was that they were "an infection" and nothing had been said about the Locust at all. Except, you know, if you were actually paying attention during Gears of War 2.
Sure, anybody could put two and two together and see that the glowing, exploding Locust were connected to imulsion somehow. And once the imulsion slithered up the wall after killing a lambent drone during the assault on Nexus it didn't take a genius to realise it was alive. So imulsion infects the Locust? Can it infect humans? Oh wait, that's exactly what Rustlung is, "imulsion sickness". So imulsion is alive and infecting everything. Hmm, maybe that secret medical facility where the patients seemed to be suffering from the same symptoms as Rustlung long ago might be relevant here? What happened to those guys? Oh yeah, they all went up into a mountain where the centre of Locust civilisation is located almost a century later. Must be a coincidence... Oh well *revs chainsaw*
That is all in Gears of War 2. By putting the pieces together a credible idea emerges. But the concept of the Kantus being an ancient race, while definately, undeniably possible, is neither alluded to or even suggested at all. Nexus is older than New Hope? Okay, so the Kantus built it? Or maybe the Mole People built it before they were wiped out by the Kantus? Oh wait, maybe the Mole People were the original humans who expanded and moved to the surface and then after the Kantus came from space and the war they later called the Age of Armageddon the surviving Kantus fled underground and inhabited the ruins of Nexus until history went full circle and the humans who later became Locust from New Hope found them?
... It's just as possible as the ancient Kantus theory. When there's nothing in the series that supports or denies it, anything is possible. Maybe Gears of War: Judgment will shed some light on the Kantus and I'll be right or wrong, or maybe nothing will be said about them at all. I keep hearing the same thing again and again and again - Nexus is obviously older than 100 years, therefore the Kantus built it. Why? Why is it obviously older than New Hope? Why did the Kantus build it? If anyone can source something, anything from anywhere in the series then great, I'll happily concede ignorance and gracious defeat. Until then, I must firmly continue to reject the idea of the Kantus being anything other than another strain of imulsion mutated human, a tangent that evolved in a different direction.
There's a quote from the Slab that stuck in my mind ever since I first saw it;
"And imulsion did cause mutations in humans. He knew that all too well now, as the COG had known for years. But the process by which it changed from an apparently biologically inactive fuel to a live pathogen remained unclear, as did finding parallels between imulsion’s teratogenic effects and its behaviour as a pathogen. Adam suspected they weren’t linear stages of the same thing but evidence that imulsion was evolving, diversifying just like the first life on Sera had done."
- Adam Fenix’s thoughts (The Slab – Chapter 13)
That one line explains everything for me. Kantus, Boomers, Drones, all come from a single source but diversify as imulsion explores different avenues of evolution. Look at the drastic changes the Brumak undertook when it went Lambent. Original animal + imulsion = wildly unpredictable outcome. This is why all humanoid Locust, yes, even the Kantus, can trace their origins back to New Hope.