Well, no work of fiction ever happened. People today still love The Final Reflection and My Enemy, My Ally and Federation even though they "never happened" in the current canon (which never happened either because it's all made up).
A good story will immerse you in its present, so you'll buy into what you're experiencing in the here and now and not worry about what might or might not happen in its future, or whether it connects to anything outside itself, until afterward. The connections in something like Trek Lit or the Marvel Cinematic Universe are meant to be a bonus, not the sole purpose of the exercise. The fact that the parts are connected would be of no value if the parts themselves weren't able to stand on their own.
I think the thing is, once you get the long-form interlinked stories happening, (sometimes too drawn out) as we got in lit trek, it becomes much more… depressing, when they end. Especially when things haven’t been wrapped up. Older Trek books were much looser than current, and maybe you would get a trilogy or similar by one author, but it wasn’t as much of an investment as readers. (I kept buying DS9 books and TNG to see ‘what happens next’ long after I lost interest in a lot of the aspects. It’s how they sell the books, and something I finally really regretted here at the end.)
One of my favourite Trek books is Ghost Ship. (I also like children of Hamlyn, and Metmorphosis) It is completely out of any kind of continuity because of when and how it was written, but that’s ok, because it’s one book, not a decades worth of reading (or writing) it’s a fun piece by itself.
For me, a lot of the lines had less and less books that were enjoyable in themselves, and now, the whole thing has felt like a waste of time.
Critically, more than it would have had we not had Coda. Because then I could blame circumstance for not having an ending, than having one that for me has exemplified the flaws in the series rather than the positives.
Trek lit (and to a lesser extent my flirtation with SW novels back in the day) has really affected how I approach TV Tie in fiction now. I was brought back into the fold by the Destiny Trilogy, but for me as a reader it has been diminishing returns since then, with only really the Voyager books holding regular interest. (And that had a rushed ending all of its own.)
I was often excited by the ideas of the little trilogies, and sometimes they were really good and I really enjoyed them, but after a while there was just… underlying fatigue. Things not adding up between authors/stories. Shock tactics that arguably started with the destruction of DS9, or the never ending malaise of section 31 stuff. The practically regular corridor fights, Worf’s girlfriends dying, jaunts to the mirror universe…
Ironically, the things a lot of people don’t like in the new TV incarnations — retcons, violence, misery, unwanted small sequels — started here in Trek lit I think.
The fact we already know there is a Jadzia era DS9 novel coming out shows the direction is moving back to something like we have always had with TOS novels, which is contained stories that can’t suffer from that. (Though I prefer movie era stuff, and am particularly fond of the very few TMP influenced books)
Maybe expanded universe Trek Lit works best when it is framing itself between things, rather than trying to reach out untethered into a future continuity.