I don't really understand what "speculative" adds, either. Maybe DuskyRose and I are missing the brain neuron path that makes that make sense. Seems like all fiction is "speculative" to me: What if Russians nuked America? What if society fetishized teenage pregnancy? What if a magic school existed and a kid with glasses attended there? Etc. (Deathlands, Bumped, Harry Potter.)
I'm not much for the "let's write one book, and stretch it into three" thing lately. I've read a lot of YA that's fallen into that category, including The Maze Runner. Then again, I've also loved a lot of good YA trilogies lately, including The Hunger Games.
So seems like this should be less a rant on series as a rant on bad writing. Just my two cents.