Do you have any idea what would happen if anyone, for any reason spoke up and said "We destroyed the STC to keep it out of enemy hands!"
The Admechs would slough off ten thousand years of stagnation and tradition soley for the opportunity to invent devices capable of recording the very ideas they would come up with as they tried to invent new and innovative ways to torture those poor fools. And thats just to start.
No, better to let the STC fall into the wrong hands and sacrifice the entire galaxy in war to win it back then to destroy it.
Countless astartes, the skitari legion, the entirety of the titan fleets, every vessel, every world that holds the machine god sacred would put aside all current conflicsts and march at even the slightest proof of a complete STCs survival.
Do you know why the admechs don't like alien technology? It's because they believe that once upon a time mankind knew all there was to know, and that anything else was dangerous. Whats more, mankind put that knowledge in a box and sent it out to the stars. That knowledge lies intact within a complete STC. Perfect and Holy. All that could ever be known, the eternal glorious proof of the perfection and dominion of mankind, safe and sound within the STC, waiting to be redicovered.
To the admechs an STC is as though someone had taken the very Omniscience of God and copied it in perfection. Each patten discovered a reflection of the glorious whole, a proof of the divine. The STC isn't some tool or relic or even holy icon to them. It is their God in floppy-disc form, the perfection and proof of perfection of the greatest of machine spirits, the most high divinity of the machine. It is the face of god.
There is, i think, no limit to what the admechs would do to get their hands on one, from the lowliest enginseer to the highest magos.
Telling them you broke it is not a good idea.