In White Dwarf issue 519, there's an article, titled The Golden Throne, about the 4-mile wide and half-mile tall "golden throne" that houses and sustains the Emperor's remains in Warhammer 40,000. It's a short article in 3 parts that cover the throne itself, Sanctum Imperialis where the throne is located, and the Adeptus Custodes who guard it. I'm an Adeptus Mechanicus player, so I read the article from that viewpoint, and as I read I started to realise that the Cult of the Machine God surely must view the golden throne a lot differently than most. In fact, I think it's no less than the fulfillment of prophecy, the ascension day of the man who claimed to be the avatar of the Omnissiah into his final state of, functionally, a component of a living machine.
Historically, the Emperor of Mankind got the Priests of Mars on side by convincing them that he was an avatar of their Machine God. I think it's pretty universally felt, at least among fans if not inuniverse, that the Emperor was prepared to say pretty much anything to get these absolute wizards of technology to swear fealty to him. The Martian cyborg cult was the civilsation that knew, mostly, how to work technology, and they literally lived only to serve technology. They worship technology, believing it to be motivated by a literal Machine God. The Emperor needed tech. The strategy pretty much writes itself.
For some thousands of years, Cult Mechanicum laboured dutifully to serve their Machine God, allowing that some commands came from the [still living, back then] Emperor. It was probably pretty neat to have a living embodiment of the Machine God. Maybe it was a little disappointing that he wasn't made out of cogs and wires, but then again he didn't seem to age and he was also conquering most of the galaxy in his own name. The Cult was being handed worlds to convert to planetary forges, they were getting to make deals with noble houses to service Imperial Knights, dveryone was gainfully employed. Everything was looking pretty good.
When Horus betrayed the Emperor, it tore the galaxy apart, and that includes Cult Mechanicum. Some magii split off from the Cult and formed what we call the Dark Mechanicum, and there were effectively civil wars where before there had been probably no more than internal power plays. I imagine that things probably looked pretty dim even for the most faithful of the Cult.
And then the Emperor is struck down. Effectively dead, he's placed on life-support. I don't know when it would have occurred to the Cultists, but that sounds suspiciously like ascension. Split from the corpus of the Omnissiah to herd humanity together, the biological representation of the Machine God suddenly returns to a machine. The White Dwarf article describes the current state of the Emperor as:
For ten thousand years, the Emperor—or what remains of Him, at least—has sat immobile upon the Golden Throne
The standard preface to nearly any 40k novel starts with an almost ritual re-telling of the establishing lore, and calls the Emperor a "carrion Emperor."
In other words, whatever biological remnants remain of the Emperor are neglible compared to the golden throne itself. The Emperor is, functionally, no longer human, or at least no more than any exalted Magos, and instead exists in the purest material form possible. He is machine, driven by a lingering Terran intellect. In His death, He confirmed the path already taken by Cultists for thousands of years.
The Cult exists largely to maintain, and recover, machinery. The Adeptus Mechanicus believes that all knowledge and technology has already been developed. No innovation is required, because the divine design is perfect. What has been lost must be found, and what exists must be maintained through prayer and ritual. (Their rituals, of course, would look a lot like systems administration to us 21st century folk.)
It must have been a little inconvenient for them to have the living Emperor hanging, without so much as a token mechanical limb for them to maintain. How do you even worship a god that has no wires or servos? And how exciting it must have been for them to suddenly be gifted, one dark day, the privilege of maintenance on the golden throne, a life-support and computer supercomputer with a 4mile footprint.
The golden throne is essentially a monument to permanent death, but for as much as the Adeptus Mechanicus must adore it, they don't appear to be a death cult. I think for them , the grolden throne is a testament to eternal life.
I have yet to read all of the Cult Mechanicum holy texts (they're on my reading list, I promise), but I have to wonder what's next for them, or whether this is it. I suspect this is it. Sure, there's archeotech yet to be recovered, but even were they to reach a state where they felt confident that they'd recovered it all, I think the infinite loop of administration is what they want. However superiour to flesh machinery may be, it does seem to require regular maintenance. The golden throne, I think, is the promise land. It's the continuous integration and continuous development cycle that provides them with purpose. Endless war is background noise to the infinite demands of the living dead Machine God.