Given the financial returns of franchises, it's no surprise that film-makers (or more appropriately, STUDIOS) always have one eye on the possibility of turning any film into a series. The potential for committed returning audiences is far less of a gamble to go for than expecting something unique and innovative to work, after all.
With that in mind, you'll see film-makers going with open-ended climaxes (or at least open-to-interpretation ones) or leaving prominent plot threads dangling in the vain hope that enough people will see the original to make a sequel a viable option. It's not always a great idea, of course, because cinema audienes can be a fickle lot and those threads end up left dangling forever, but you can see why they'd do it. Who wants to have to write a whole new script, after all?
Sadly, though, because of having one eye on what comes next, those film-makers don't have both eyes on what they're supposed to be doing and the sequel set-up ends up being redundant when the first film fails for some reason. And then we're left with sequel-baiting that leads into a dead-end.
Because, unfortunately, the best laid plans of mice and film studio men often go awry...