It was because after Scott Snyder changed the cosmology, The Monitors of Nil and consequently The Thought Robot and Mandrakk became irrelevant. There should be two profiles for the monitors: the original Grant Morrison version and a Scott Snyder cosmology key as they are radically different. In reality, as much as people in the debating space like to go on about The Thought Robot and Mandrakk, they only in 2 comics and did not have that large of an impact on the overarching plot of DC. Since their original appearance, there have been only a handful of references to them.
Yeah like i've told earlier, cas isn't really necessary. Even after cas destroyed, multiverse still survive a greater threat than final crisis or mandrakk, which is TDK in death metal event. All this because of they're actually alive or embedded in reader's mind. This reader's belief(actual reader in real life)of "To be Continued" rule how the stories will be written. So rather than actual plot manipulation for cas, i find it cas was assisted by nature of story itself that will never make the multiverse being destroyed, so these multiverse or characters will always have a stories across decade outlast their past writers. Which is why nix reply to mandrakk like this :
"This multiverse has natural defense none of you could imagine"
After mandrakk saying "i saw the book of limbo, everything was all flame" in the end of final crisis (after cas destroyed beyond repair).
"Im living in your head(reader). As long as there's one of you(reader), there'll be a hundred of us"
"Unmaking DCU to the point of destruction & showing how its internal rules homeostatically reset it, Superman always saves the day or he's not Superman. Its a self perpetuating idea"
"Its another way to say on this relentless idea of realistic superhero comics, because the 'realism' in this one comes from the reader's acknowledgment that what he holds in his hands is a section of a real, alternate universe of superheroes, living out lives across decades"
This self perpetuating idea of superman makes character Superman have stories that always continue until today, so the story of superman is indestructble.
"First off, you've spoken about the power of story, and
the power of the DC Universe in particular, and expressed a certain level of awe at how these creations and these stories outlast their creators significantly.
This sentiment seems to play out directly when Zillo Valla faces down Mandrakk's dream of the ultimate void by telling him she
found a story in the germ world that's unstoppable and indestructible, about a child rocketed to Earth from a doomed planet. Is that pretty much what you were going for there?"
Morrison : yeah, pretty much.
I have written this many times, but there are still who don't understand and asking "how story of superman grant Superman something" uhh. Indestructble story of superman means character Superman always have a story, so any narrative saying he's dead/destroyed are false.