There was a time, oh so long ago and well before my birth as well as for a while afterwards, when a self-respecting writer of imaginative fiction—science fiction, fantasy, fairy tales—could get away with writing endless disconnected novels, and he could earn a respectable living and reputation from it. I said 'could get away with', because while there are plenty of examples of those who did, there are also others who either did write 'series' or else at least connected their novels in some form, as Heinlein did, though only retrospectively, when he joined a lot of his books with the Future History framework; and that was actually a creation of his editor. Then came the whole series involving, in some form, characters associated with 'Lazarus Long', starting, I suppose, with Methuselah's Children.
Heinlein wasn't alone, of course. There was Frank Herbert, for example, but later. And Jack Vance also wrote some series, like Planet of Adventure and later, the enchanting Lyonesse trilogy. And in Germany, the writer Karl May wrote what amounts to imaginative travel-adventure stories long before any of these guys, many of the connected by the characters appearing in them and thus qualifying them as 'series'. And there's Tolkien, of course and, yes, Asimov and Clarke. And then there were science fiction pulps, especially successful in Germany, like the Perry Rhodan series; but these were usually written by a whole collection of authors. As an aside: I spent several years of my youth reading Perry Rhodan pulps, and possibly read up to 500 of these things over a period of ten years; 52 issues/year, 1/week—much to my parents' dismay. It wasn't exactly 'high' literature.
Still, for the comparatively few series by single identifiable authors, there were a gazillion writers who didn't write series, and many of those mentioned above also wrote many books and short stories that weren't parts of series. Meaning that 'series' were, by and large, exceptional.
The situation today is significantly different. Series are all over the place. Not that I'm dissing the phenomenon; glasshouse and stones and all that. Just making an observation. It has also occurred to me that in today's publishing environment there are excellent reasons for pursuing this course of writing. Newly published books by and large have a fairly short shelf-life these days, even those by A-line authors and especially if they're not in what you might call the 'literary' category. Single-volume works thus look forward either to raging success or being taken off the shelves and not replaced by publishers within a year max. If there's a hardback edition followed by a paperback one that may stretch, but not necessarily by much. This is even more true for authors with less pulling power.
Serialization on the grand scale is an expansion of the process as it was implemented in sci-fi mags years back; only now every installment is a whole book. Spreading 'serials', a.k.a. 'sequels', over a period of years effectively keeps all the books in the eye of the interested public, since those who find later installments are likely to want the previous ones, and in order to sell said sequels the availability of the previous ones makes excellent business sense, especially if the series is 'tightly connected', that is one would have a difficult time understanding the sequel without the preceding books.
Now, I'm not spreading the Tethys series across years; in fact they're all available simultaneously, but then again I'm not in the position of having a marketing organization making he public hungry for my work. Presenting the whole story in one block in this case is probably the best policy. Dance of Tigers is a sequel-to-come, with possibly the best part of a year passing until it's done and ready, and with my other projects possibly later, but I think it's necessary to establish some sort of credibility, and so...
Anyway, the trend is clear, and if series are 'it' then so be it. Not that it comes hard to me, and anyway, there's stories you can only tell if you have the space, and humongous tomes aren't really my line.