It's a truism of stories. By the time we get to Happily Ever After, the characters aren't interesting any more and drop out of the story. Telling the story of someone who lived happily ever after isn't what we do. In fact, they're as good as dead. We can reminisce and remember, but we can't interact. There's certainly no struggle.
I wonder whether this idea was also in the consciousness of messianic authors. The visions of a religious utopia, presided over by God (and / or his appointed human agents) sound much like a happily ever after, and to my mind, an everlasting death. Although the general conditions of such a utopia are discussed (lambs sitting down with lions, etc.), there is very little other detail.
We can read those passages as incomplete, intentionally or otherwise. If the author didn't mean to leave out the details, then they just weren't being exhaustive in their work. If the author deliberately left out those details, they've made an assumption about their readers.
Another way to read them would be as aspirational. The author has written about specific features of life that they would like to see changed. We can treat them as a giant Ought, limited to a specific issue or circumstance. The blanks are still blank, however. Even Marx has been accused of this. He critiques the capitalist system nicely, identifying its failings, but when it comes to proposing his alternative there are still some features missing. Someone still needs to collect garbage and clean toilets - and if we have machines for those tasks, someone still needs to clean and repair the machines that collect garbage and clean toilets.
Happily ever after is the same as death. No matter who's writing the story, the moment we remove the struggle we may as well stop telling the story.