Indiana Jones is old. Not just as a franchise (it's been around since 1981) but also the actual Dr. Indiana Jones himself, Harrison Ford (he hit the big 7-0 this year.)
Indiana Jones and the Kingdom of the Crystal Skull brought along with it swinging monkeys and giant-sized ants, but that, coupled with Jones' prowess, wasn't enough to bring the series back to its original glory. But rumors about a fifth installment persisted anyway, especially since Ford pretty much said he was still up for it (no pun intended) and Spielberg seemed open to the idea.
Crystal Skull was pretty much a stretch in many respects and producer Frank Marshall agrees:
I say, for me, [Kingdom of the Crystal Skull is] the last hurrah. I know that yes, we talk about it, but there’s no idea, there’s no MacGuffin.
I'd say it was a pretty lackluster hurrah but at least the movie didn't totally bomb. And with George Lucas's announcement that he was leaving Lucasfilm to make "experimental movies" in his garage, I'd say he's done with Indiana Jones, too.
Weigh in: Should the Indiana Jones franchise end with Crystal Skull? Or should they push for Indiana Jones 5 and hope it fares better?