Sigh.
Ok, fine, so I only watched that cutscene once. Whether the Chief was sprinting, walking, or breakdancing toward the Didact, the fact remains that he was carrying a nuke the whole way. Didact's reaction is not "Get that thing away from the Composer", but "Hold the Chief up and launch into yet another cliched monologue". Don't get me wrong, a good villainous monologue can be truly a thing to behold. When it's not... well, it can make the villain look like an idiot. Especially when he forgets some very basic precautions.

Maxim #8: Mockery and derision have their place. Usually, it's on the far side of the airlock.

: Since the dawn of history, there has been bad writing.

Vagueness, how am I supposed to take the above as anything other than a blanket excuse for lazy and incompetent writing?

: That kind of depends how you look at it. In gameplay people died because of
: them a lot. Canonically, sure, he survived all of them, but who can say
: how narrowly? How can we know, especially with varying difficulties, how
: difficult it was for the Chief, and how many close calls there were or
: might have been?

The fact remains that the Chief got from point A to point B, and there was an army of Covenant and Prometheans in the way. At some point in time, the Didact has to change up his tactics.

: side note or question, is Heroic still the "most
: canonical" difficulty?

It depends.
Personally, I like to think of Legendary as representing how tough the Elites are. But there's a lot of stuff that needs to fudged for every difficulty level, like how Grunts can take ten 7.62mm rifle bullets to the forehead before dying.

: Yeah that was weak. I have a feeling though that 343i did that more and
: people yelled at them after falling to their deaths, so they scaled it
: back until it was almost gone to keep that section fun (or fun enough).

I really wouldn't be that surprised if the guys at 343i were that unimaginative, really.
OK, you want the Didact to destroy and really present an obstacle? Have Cortana shout out a warning. Have the player run for the safety of a waypoint she lays out. Render the area impassable, and have Cortana think of a new way to get to their destination.