Maybe we are thinking about this in the wrong way. I have trouble rationalizing this speech to occur at times very close to the Halo firing (which is the specific time frame most are arguing for or against compared to post-Halo 4), but maybe the speech was set further in the past - prior to the Ecumene Council being destroyed, prior to the Flood absolutely stomping the Forerunners around the galaxy.
I'd like Catalog to explain the exact time & location of this speech, because I'm starting to think that it occurred when the Ecumene Council sentenced the Ur-Didact to his exile in the Cryptum.
In this hour of victory, we taste only defeat. I ask: why?
We are forerunners, guardians of all that exists. The roots of the galaxy have grown deep under our careful tending, were there is life the wisdom of our countless generations has saturated the soil, our strength is a luminous sun towards which all intelligence blossoms, and the impervious shelter, beneath which it has prospered.
I stand before you, accused of the sin of ensuring forerunner ascendancy, of attempting to save us from this fate where we are forced to...recede.
Take this in context immediately following the Human-Forerunner war, when the political fights were going on between Warrior-Servants and Builders.
It was revealed to the Forerunners that humanity had been fighting on two fronts - against the Flood. The Forerunners thus engaged in a lengthy debate how to counter a possible re-emerging Flood threat. The Ur-Didact wanted to store Humanity's memories period (similar to Forthenco's was). However, de-evolved human populations were permitted mostly because they were perceived to have found a biological 'cure' against the Flood, and thus live human populations were needed to study this.
Around the same time, the Lifeworkers (especially the Librarian), began to come to the conclusion that Humanity was expanding throughout the galaxy and acting as caretakers - upholding the Mantle.
Additionally, the Ur-Didact and the warrior rate lost a huge political battle in endorsing a Shield-world based strategy to combat the Flood; the Builders, with their massive Halo projects, usurped power, and most warrior-servants ended up builder-security. His plan abandoned and he was forced into exile (Cryptum).
Rather than supporting an offensive plan, to eliminate a biological human presence (who in the Ur-Didact's eyes were contesting the Mantle and thus Forerunner ascendancy) and to effectively contain the Flood (Shield World strategy), the Builders won, creating the galaxy-wide equivalent nuclear option (the Halos), and human populations continued to exist and be supported, though de-evolved. I believe the original Halo plans were intended to be used for tactical pulses only, but they still understood that it killed all organic life - which is surely something that the Ur-Didact believed to run against the Mantle and against Forerunner ascendancy to the Mantle.
Humanity stands as the greatest threat in the galaxy. Refusing to eradicate them is a fool's gambit. We squander eons in the darkness, while they seize our triumphs for their own. The Mantle of Responsibility, for all things, belongs to Forerunners alone.
Similar themes to what I posited above. Rather than having all human's personalities/memories archived, humans survived. Ur-Didact is now sentenced to exile in a Cryptum, and his plans disbanded. The second line could be referring to the Ur-Didact's perception that Humanity had attempted to seize the Forerunner's triumphs (upholding the Mantle; supremacy) for their own, and the darkness was the prolonged Human-Forerunner war, and the ensuing fallout (both logistical from losses of colonies, people, resources; and political, from the upcoming waste of time that he perceives the Builder plans to be). Thus, present tense, because he views the actions the Forerunners are taking against the Flood to be incorrect and against their ability to uphold the Mantle that was just contested by Humanity.
Think of my acts as you will, but do not doubt the reality: the reclamation has already begun, and we are hopeless to stop it.
This quote, stated post-Halo 4, only makes sense if the reclamation is occurring by Humanity.
Possible ways it can be interpreted, if stated immediately after Human-Forerunner wars, but prior to the Ur-Didact's Cryptum?
Forerunners trying to reclaim the Mantle after it was contested by Humanity? - meh, because "hopeless to stop it" really doesn't fit in this syntax.
Humanity - doesn't fit at all following the Forerunner victory against humans.
But what about the Precursors/Flood? For the Ur-Didact to even imply that the Flood would soon contest the Forerunners by reclamation, he would have to know that THEY were actually the Precursors. Indeed, the Ur-Didact met the Primordial at the end of the Human-Forerunner war, and it took thousands of years following the end of the war for him to finally be ruled against.
So this last sentence, in combination with the above, is a really bitter narrative by the Ur-Didact - someone who won a massive war against Humanity, only to realize they would face an even bigger threat (the Flood); whose plans to uphold the Mantle and guarantee Forerunner ascendancy was being ignored and cast away for plans that went against the Mantle, and would further not be adequate to protect against the Flood, who only he understands - the Flood are reclaiming their place of dominance in the galaxy, and the Forerunners, led by the ignorant Builder rate, are hopeless to stop it.
Thoughts on this?
Lastly, please don't chastise me for blindly trying to fit the narrative to what I want. Prior to Catalog's statement, I had assumed that Didact's speech at the end of Halo 4 was actually set within the Halo universe post Halo 4.
I'm merely trying to think a little differently now and play devil's advocate in the hopes that we might find a new way to re-interpret the Ur-Didact speech, or Catalog's statements, or both.