It already has been, just not by the knights. Shadow lairs. The Sanctuary.

@Noklip
How does that imply either of those things? Cradle was (originally) built by the Architect to protect the contents of the Core from the Swarm, before it became a "game of death." See Echo Stone: Ironclaw Munitions Factory:
This world was created to protect me.

Vulnerable in slumber, worlds were drawn to shield me from the darkness.

This world has become my prison.
In his madness, the architect betrayed the pact. Twisting his creation into a game of death, it now devours all who enter its iron embrace.

Also noteworthy, the text of Recon Module: Entrance to the Core:
A Recording from Parma, Alpha Squad Recon:

We are at the end of the line. Pursued by some kind of terrible darkness we are forced into the unknown.

As we neared the Core- I don't know how to describe this really... the shadows came alive and began to attack us. Rulen became enveloped in it and by the time Grantz freed him his body was badly burned. He said it felt like both fire and ice.

And then we could go no further. As I record this, the darkness is nearly upon us, comsuming the world we see, the light.

But then the Core, it began to open, filling the place with such a light, such and intense, blinding light. Euclid has made the call; faced with the darkness, we'll enter the core, enter the light.

This will likely be my final recording.

Now tell me this: What happens in the end of the Unknown Passage (especially the last "room" before the lift), and where do you end up after taking the lift?

@Noklip

Fair enough. Apologies if that came across as rough, but I get a little feisty about this topic when I constantly see posts of "when is the core going to open??????" by people who don't take the time to look at what has been presented to them. Basically the entirety of the Unknown Passage is a trip through a broken down clockworks level that is completely infested with Swarm monsters. The final "room" is wave after wave of increasingly difficult fights where the only real goal is survival, until eventually a gate opens behind you where you make a mad dash to an unmarked elevator and end up in Sanctuary. It all fits very well with Parma's description of their 'rescue.'

When you enter Sanctuary a disembodied voice speaks to you and says you are safe, but to prepare for further fights as the worst is yet to come. The disembodied voice seems to imply that the Core is sentient and opened itself for you and the Alpha squad. The Spiral Order is looking for a way to essentially force it open from the outside*, something that hasn't been managed yet.

Once you leave Sanctuary, you are deposited at the Entrance to the Core terminal at depth 29 (worth noting that you are standing on the seemingly functionless elevator pad in front of the Core when you come up). The only way to go from there is up the elevator back to Haven.

The only real problem with this theory is that it doesn't offer an explanation for what happened to the Alpha squad post-core, since your squad resurfaces but theirs apparently didn't. My only thought is that they 1)chose to stay in the core (unlikely, since they aren't in there when you arrive at Sanctuary), 2)resurfaced somewhere unknown and are currently stranded in the clockworks (possible), or 3) are continuing to fight the Swarm at the Sanctuary voice's urging (in my opinion the most likely).

*I also have a separate but connected theory that the Spiral Order are the true villains of the game, but that's another story for another day.

@Infernoburner
If you'll recall, everything in the clockworks is constantly moving and cycling. One could argue that a platform's current location is irrelevant when looking at something like that. When the Alpha squad was on that platform, it could have been somewhere else entirely and connected to an entirely different series of platforms. There's also no reason to believe that there is only one Unknown Passage (or even that there is only one Sanctuary--if the Core is supposed to be the size of a planet core, then it's probably pretty big inside).

Now if you are legitimately arguing that Sanctuary isn't in the Core, then... where is it? If it is just outside the core--where the Swarm infestation is the strongest--then why hasn't it been corrupted yet? Why do the events of the Unknown Passage match Parma's description if that's not the case? Counter arguments > nit-picking arguments, if you are trying to make a point.