Insofar as it goes, it's true. Outside of Neal Stephenson, I'm hard pressed to name a one-off I've read recently. Traditional (mostly Baen), or Indie
Another major stumbling block is authors painting themselves into a corner, or worse, losing control of the series overall arc. Simply not finishing it - or looking like it never will. As has been discussed ad nauseum regarding George RR Martin's Westeros books, he long ago wandered off the trilogy reservation, is still adding more plot threads and characters, and doesn't seem on track to finish the books in his lifetime. The TV series has eclipsed the series which started twenty years ago.
You know what I loved about Babylon 5? It had a beginning, a middle, and an end. Even Gary Larson and Bill Watterson quit before their work became a parody of itself.
John Ringo has how many series now that have just petered out and not come to a conclusion? Granted, they're all awesome, and he writes so fast you lose track of what was left behind as you buy the next hit of black ink crack. It's also true he's at least found good stopping points for many. He's mostly resolved the Council Wars, thank god the Prince Roger books found a good stopping point, and The Black Tide books came to a sudden - and clumsier than usual, as much as I loved it - but appropriate halt, but the Aldenatta storyline is in cliffhanger mode, Troy is in limbo, Harmon and the Keldara need a new mission, and holy crap he's writing in Larry's universe?
Series have one other nasty effect - even on authors that are utterly outstanding, and haven't lost the thread.
OK. I love Larry. I love Ringo. They're fantastic storytellers and their writing has only improved. I want to find out what happens to Owen, Milo, Earl, and hell, even Franks - and I'll stand by my opinion that as much as I like all the MHI books, Alpha and Nemesis just work better. If all Larry ever cranked out was MHI and Grimnoir, would we see "Son of the Black Sword"? As much as I want to find out what happens to Tyler Vernon, or Prince Roger, would I want to sacrifice Sophia and Faith? Or Chad?
Ringo will wrap up or simply drop the series until the muse attacks him in a dark corner again. And even his muse is hard pressed to keep up with the torrent of different universes he generates.
Larry dodged this also - while there will be more Grimnoir books, they won't be properly speaking sequels, but further forward, a new generation, new people, new stories. MHI is still effectively open ended, but he's now free to explore new territory and ideas.
If they stayed in their current worlds, they could never tell stories that cannot be told within the existing series.
So some stories need more than one book. Many don't. Authors should give themselves the opportunity to try something new, even as they forge ahead with what they know works.