Every generation is only about eighteen years of people being born and they all have about the same attention span with older people developing somewhat longer attention spans that come with age and younger people having short attention spans that come with brain development and lots of entertainment content often aimed specifically for them.
Way back in the 1990's, for instance, predictions were made that short story collections were going to be the new in-thing because newer people then supposedly had much shorter attention spans, didn't want to read long novels and so would prefer short fiction they could digest quickly in audio, commuting, electronically, print, etc. Except short story collections did not become the new in-thing then. Giant ass novels did. Which many other readers don't like. The short attention span argument gets made every decade for predictions and remarkably does not develop much in the way of trends, nor do scientists really find it happening. It's a social perception that results from aging, which seems to involve disapproval of how younger people do things.
Likewise the predictions thirty, twenty years ago was that the younger generations needed to push buttons with pictures on cash registers and watch videos on the Web and couldn't do math fast in their heads and so they would become largely illiterate. That wasn't true either. The younger generations write and read more, thanks to the Internet, than any other ones before them. They also don't act as a monolith -- they have different interests in different forms of story-telling. So I tend to throw cold water on generational assessments because they don't tend to pan out.
Which is why you're going to be disappointed that serialized novels have not died out as a tradition in SFF. It still goes on in magazines some times and it's become a successful form in self-publishing/electronic publishing. As I mentioned, Howey shot to fame doing it with Wool. While his novellas were somewhat episodic, they are all parts of the same story, one novel. Other people are still quite into the did little Nell live thing. And in comics/graphic novels, both online and in print, it's still very much a common form. All sorts of funky experiments are going on, from audio podcasts to new shared universes.
If there's a format, some authors will play with it. Nothing dies out. I mean, nothing. Something may not be super popular and have a growth expansion for a bit so it might seem like it's gone but it's deceptive. People are actually doing it and sometimes hits come out of it. So we'll always have trilogies. But we'll also always have standalones and quintet series and so forth.
