I don't think that's the issue, but rather that the characters you know from tv have changed since you saw them last time, 12 in-universe years ago
Yeah, but it's the same principle. Once you get past the initial "This isn't what I expected" phase, then you can learn what the characters are like now by just reading the story, just like you can learn about brand-new characters in a series premiere or standalone work by just reading/watching the story.
For example, look at
The Wrath of Khan. The characters had suddenly changed massively from when we'd seen them last. Kirk's an instructor at the Academy? Spock is captain of the
Enterprise? Chekov's first officer of some other ship? Kirk has a
kid? And who's this young Vulcan woman they all seem to know? When did all this happen? But we got to know the new status quo just by watching the movie. We didn't need prior knowledge of how the changes had happened. The story caught us up. We were confused at first (indeed, the film actively used that confusion in the opening scene), but we soon got the hang of the changes.
Again, the ideal is to write every story as if it's someone's first. It doesn't matter if it's a continuation of a series -- you still want to make sure it contains all the information the readers need to follow the specific story you're telling. If some bit of background isn't included in the novel, then it isn't important to that novel. At least, that's the way it should be.
Doctor Who is the perfectly wrong series to use as an example. The cast change you mention is baked into the premise of the show and every single story is a standalone. There's literally zero continuity to worry about.
Not necessarily. Imagine what it was like for me seeing the opening scene of "Robot." It just picks up in the middle of an ongoing scene, even in the middle of a line of dialogue -- "Now, just a moment!" There's a military man and a young woman, and the woman says "Look, Brigadier, look. I think it's started!" and then we cut to a white-haired fellow unconscious on the floor, but he then immediately turns into a younger, curly-haired fellow, and the man addressed as Brigadier says "Oh, well, here we go again." That's a pretty confusing scene to be your introduction to the entire franchise. So it took me a while to catch up and understand what was going on. I had to learn as I went. (And it didn't help that I never saw the inside of the TARDIS until six serials later.) But I still got the hang of it even so.
So to defend Star Trek novels becoming serialized fiction, despite the TV series being overwhelmingly standalone, you're pulling from a few other serialized franchises as proof that it works. Yes, serialized fiction works. That's not the question. The question is whether Star Trek, an overwhelmingly standalone franchise works as serialized fiction. From where I'm standing, it doesn't. Because it no longer feels like Star Trek because of its now serialized nature.
That's not what I'm trying to demonstrate at all. I'm just saying that it's possible to experience a series out of sequence, or to jump over a large portion of it and pick up at a later point where the cast and situation have changed, and still be able to catch up. There are a lot of shows where I've come in midway through and then gone back to see the earlier parts, or dropped out for a length of time and then come back after things had changed. There are novel series that I've read out of order because I couldn't track down all the books at the same time. And some of those have been very serialized, others more episodic. It's not ideal, but it's doable.
To make the novels mostly serialized is to intentionally write them in a different way than the series, and for some fans, it's a decision to exclude them as part of the target audience.
Except that, again, there are still standalone novels and novellas being published. Yes, sometimes they're set at a later time after the cast has changed, but their individual plots are still self-contained and understandable. Pocket's goal has not been to exclude any audience, but rather to offer a range of different types of story to appeal to a range of target audiences. Not every work will appeal to every reader, but ideally there will be something for every taste. Admittedly, there haven't been a lot of 24th-century standalones in recent years, but maybe that's a lack that will be made up for in the future. There's no deliberate agenda to be exclusionary.