Once upon a time I was 13 (around about 1978 if you must know) and I started reading the 'Lensman' series by EE 'Doc' Smith. It was my first introduction to the big galaxy spanning space opera style of novels and being honest its shaped my tastes to this day. The worst moment of reading it was when I finished the last book and I had to face the awful fact that I would never know what came next; that I would have to part company with the characters who had come to mean so much to me. There have been other series I've been immersed in since but I was braced for the inevitable with those and of course from a certain perspective its a triumph for the author to have drawn in the reader and leave them wanting more. Alas these days it seems all too many authors in science fiction and fantasy can't bring themselves to leave that demand unfulfilled; they will give the readers more, and more, and more, until far from fearing the end of a series the reader is yearning for it.
The classic version of this is the series where the heroes have to beat some terrible enemy and after several volumes they triumph; except of course they don't. Soon enough there's a new book with a more powerful enemy, and then another, and another seemingly ad infinitum in some cases. If you want a good example try the 'Lost Regiment' series by William R. Forstchen; lovely books but with a massive power creep among the villains. By the last book it transpires that the monstrous but essentially nomadic and non-technological enemy happen to have fully industrialized cousins on another continent who never came up in any of the EIGHT previous volumes.
Now this unwillingness to bring a series to an end is understandable sometimes; what starts out as a simple idea steadily expands and plans change. My own 'Fourth Planet Problem' story was exactly like that; it started as a short story and mutated into a full fledged novel. Where it becomes an issue that frustrates me as both a reader and a writer is where the books start to include great tracts of padding to spin out what might have one book in a series into two, or even three. This doesn't even address the current trend for branching off; that is where you have a main sequence of books and a veritable blizzard of off shoots focusing on minor characters or elements of the background of the universe but sufficiently entangled that missing one leaves you struggling to make sense of the rest, or worse they go over the same ground multiple times so the reader gets the same material again and again.
A writer has to care about the universe they are creating when writing science-fiction but there has to come a time when you write the last chapter and let it go; preferably before your readers die of old age.