No one is writing stand-alone novels any more. Go stroll through Amazon’s ebooks, and you’ll see that virtually everything you see is “book one in the Fartknocker series.”
Now obviously, that’s not true, and probably never will be. People will always write stand-alone stories with no sequels or long-range plans, and publishers will more than likely always buy them–but lord, don’t they love them a series.
That’s where it comes from, you know. People see publishers putting all their marketing and effort into this or that series, and assume–almost certainly incorrectly–that they are only interested in the next big series. Therefore, everything has to be stretched into a series, whether or not it has the potential for it. So, sadly, many great characters and stories are forced to keep shuffling across the stage long after they should have been allowed to take a bow and fade into the background for the next act.
I’m about to propose something radical. I preface this by saying that I do, in fact, have a moderately successful series of my own, although my favorite of my works remains a single, stand-alone novel. But I think this merits saying.
FORGET ABOUT THE SEQUELS AND WRITE THE BOOK.
Don’t worry about it. If the story doesn’t merit a return, then don’t go back. Finish the story, and let it shine on its own. If it has the potential to be a series, it’ll let you know. Don’t force it.
After all, imagine how lame a sequel to Dracula would have been. (Actually, you don’t have to imagine–there are countless lame novels vaguely based on what happened after the good Count lost his head.)
Tell the story. If it’s done, it’s done; don’t try to reanimate a dead story line to stretch it out.
Michael Chambers's Blog 
Surprisingly, I agree with you. Series and trilogies are taking over the publishing market. However, I feel readers like stories where they can invest in the characters and plot. While I don’t mind standalone novels myself, I still prefer long series.
As an indie author, it makes absolutely zero financial sense to write a standalone novel. Series simply have so much more earning potential, and they’re easier to market.
If one is writing for the fun of writing, one should write whatever the heck one wants. If one wants to actually, you know, try to make a living, however, series are much better.
I guess we’ve had different experiences. My biggest consistent earners have been stand-alone novels.
It seems to me that authors get their best sales from promoting the first book in a series at a discount and making their ROI from sell through. I guess that a rising tide raises all boats, but I’d think that sell through to unrelated books would be a lot less than with a series.
How do you effectively promote a standalone?