I think this episode nicely illustrates the vast gulf that exists between "Being able to plan a multi-part story years in advance" and "Being able to tell a good story".

This scene, where the Chief stabs the Didact in the eye, hearkens back to the cutscene in Halo 4 where the Chief stabs a Lich to catch himself. It was bullshit back then too. Unless the knife was forged from the bones of dead Norse gods, it should have broke. And even if it found purchase in a seam between armor plates or something, it should have snapped under the strain of slowing a half-ton Spartan from splatter speed to zilch.

But the Didact is even worse. Between the unstoppable force that is the Chief's fist and the immovable object that Forerunner armor ought to be, the knife should have shattered. Assuming, of course, that the knife gets past the armor's other defense systems. Where are the layered shield systems? Where's the point-defense energy projectors? If the Didact has telekinesis (or ultraviolet hardlight) why doesn't his armor use similar technology to punt assailants across the room? Why doesn't the Didact, as a Forerunner soldier, have the enhanced speed, strength, and situational awareness needed to catch the Chief's arm and rip it out of the socket?

In closing, Escalation #9 proves that 343i can make long-term plans and stick to them. But for all we know, the Didact losing an eye is a plot development that came about in the same process that led to Requiem getting thrown into the sun.