Controversial UPG - the doomed thing was inserted to align Norse religion with Xianity and the desert god’s love of finality and death, which I covered in another post recently.
Why such blasphemy?
The event is attested primarily in the Poetic Edda, compiled in the 13th century from earlier traditional sources, and the Prose Edda, written in the 13th century by Snorri Sturluson. In the Prose Edda and in a single poem in the Poetic Edda, the event is referred to as Ragnarök or Ragnarøkkr (Old Norse; meaning Fate of the Gods and Twilight of the Gods, respectively), a usage popularised by 19th-century composer Richard Wagner with the title of the last of his Der Ring des Nibelungen operas, Götterdämmerung (1876), which is “Twilight of the Gods” in German.
From Ragnarök - Wikipedia - that link has references for all assertions made
Poisoning the tree of life for that religion by giving it an end that is intended to mimic weird desert concepts, and then lead neatly to a “better” future that resembles Adam and Eve. 
Defacing the old king to write over his monuments with your own is a trick as old as time, as is trying to force the meme that a thing is futile, “Surrender Dorothy,” everything is already controlled opposition, everything is already fixed in place for your defeat, “if you can’t beat them, join them” because all is lost.
From Sun Tzu to /pol/ we see that same tactic used, and it’s used because, largely, it works - at least on those unaware that such seeming grim realism is just a tactic to gain control.
Overwrite an oral tradition that’s being eradicated at ground level with a written re-imagining that makes it futile and casts its gods as dead men walking, and you create a diversion in a belief system that will outlast the true tradition.
If this happened in another context, one could be dubious, but all Abrahamic cults are fixated on death, on killing their fellow cultists for thought-crimes, on mass disasters that judge the “good” from the “bad” and just end everything, and on attaining a state of death-in-life, the eternal bliss of their paradises.
And you only need to insert a tiny amount of spin into a story to make it appear completely different, in terms of the mass perception (which is, always, all that counts).
I believe Snorri Sturluson took a cyclical concept akin to that of the Hindus (and Aztecs) and worked it into a bit of (attempted) predictive programming and spin, to make the Norse religion seem weaker compared to the living “eternal” Christ.
They took, and overwrote, the Yule celebrations, marking the first day the Sun is visibly moving south again, and attempted to pass that off as the birth of their “son” deity, they also took Easter, and intentionally built over ancient sacred sites. This is not conspiracy theory.
But those sniff-tests aside, the basis of this is my own UPG and of course, I don’t expect anyone to swallow it untested.