An alternative would be an enlightened and cultured civilisation; advanced in a way so as to eradicate our more negative feelings such as hate, greed and envy.
And many (older) SF works think
that to be some kind of goal to which our "evolution" is progressing, which would be quite wrong to believe. There is no end point to be reached, and even if there was we are no nearer towards reaching it. It stems back to Darwin (The Descent of Man) and our longstanding belief that we sit at the top of the evolutionary tree, and have a soul which animals do not. Other SF works think that we can become so spiritual that we lose our bodies entirely to become Ascended Beings.
But leaving those religious aspects behind, I do agree with you. There was a time when it was thought that we could eradicate such negative feelings with drugs, and psychology, and by creating populations of sexless drug induced happy people looked after by a benevolent supercomputer; one that invariably was nothing of the kind, and had to be over-thrown.
Vulcans of Star Trek were postulated as a more advanced species because they used Logic to eradicate their more negative feelings such as hate, greed and envy.
More modern SF works postulate that we could eradicate them by having our minds uploaded to a computer hard drive, though there still seems to be a lot of conflict in such stories because a story without any hate, greed and envy would be very boring.
Which is why I asked before "what does advanced really mean" and what exactly is "iffy science fiction" that is not allowed within this thought exercise. I think we should stick to the idea of "advanced technology and knowledge".
Edit: On which basis, you're correct - the Krell are technologically advanced and could have died out just as the technologically advanced Egyptians did.
