u/DavianThulesGhost

Either the Tyranids eat them, the TSons kill them or corrupt them to be killed by us...

...or the Inquisition shows up and executes the survivors because they've now literally seen things the Imperium doesn't want the greater public to know - Traitor Astartes, daemons, the very existence of "Chaos" - or because even coming in contact with the enemy might result in them being tainted (similar to Titus in SM1) but either way.

....buuuuut I'll also say, YMMV on this: while this is a known thing that the Inquisition does and one of the reasons the Space Wolves literally fought them and the Grey Knights after the First War for Armageddon, namely the "Months of Shame" afterwards, 40k has been known to go so out of the way to be grimdark that there's no way something can be taken seriously, like "every world in the Imperium is a fascist hellhole where everyone works 20 hour days" or the number of times an Inquisitor Exterminatus'es a planet, or "a million guardsmen die every minute of every hour of every day". If the Inquisition really did kill every single Guardsmen that fights against Chaos and wins, 1. who's left to fight Chaos 2. who's left to pass on their knowledge and expertise, and 3. that would strengthen at least one Chaos God, if not 2-3, which in turn leads to more daemonic incursions, stronger daemonic forces, and so on, and so on, and so on...so yeah.

Either way. Depends on how you look at it.

The Mechanicum doesn't have enough men on the planet to make that happen. Couldn't they have sent enough? Especially with how "men" could be anything from Techpriests to servitors which are barely men at all.

The Mechanicum is just maintaining it in accordance with it being a sacred relic. If that's true then what's all this stuff connected to it and why did they happen to have all of the infrastructure to even give it liftoff (other than the Doylist reason that basically boils down to "plot's gotta plot", even it doesn't so much as "fly" as "glide"?

And even if they are maintaining it because sacred relic, does that mean every piece of Imperial hardware has its own AdMech retinue?

And yeah it is 4000 years old. You could say it's not worth the trouble considering, depending on how much damage it took, they'd have to rebuild it from the inside out. Given how little technological innovation happened until Daddy Cawl showed up, I highly doubt it'd need that much of an upgrade, and as we saw, it is mostly intact and was able to achieve flight (yes, breaking orbit is a whole other thing) so its hull can't have been that beaten up.

Given everything else happening in the Imperium right now (and I couldn't find a link directly do it but go ahead and add Magnus and the Thousand Sons, the very same mf's we're fighting here, royally fucking up the Space Wolves and their home system), you'd think anything and everything that can be returned to active service, would be. I'm sure there's another Chapter somewhere out there that could use another battle barge, even if it's still got AM/FM radio, a built-in cell phone, a tape deck, and ashtray.