This page (requires 2k to see) breaks down recent closed questions by close reason, and also shows what happened to those questions after closure. In the last 90 days we have closed 87 questions as too broad (the largest single bucket, about a third of the closures in that time). Of those, 24 were edited. Of those, 7 were reopened. That's less than 10% edited and reopened.
Worldbuilding, by its nature, attracts broad questions, so naturally a lot of those rightly get put on hold. But ideally it doesn't end there; in an ideal world the question gets improved and reopened.
What can we do to help askers produce answerable, not-too-broad questions? Do we need better guidance up front? (If so, what?) Do we need to provide better feedback in comments? Do we need to be more active in editing? Do we need to be more clear that "on hold" is meant to be temporary and the asker shouldn't give up? Something else?
Presumably some of the questions really aren't salvagable, but 90%+?