I have one bass currently in a gig bag because I'm out of wall hanger spaces. My 1960 Precision bass is on a wall hanger (high quality, known to be non-reactive to nitrocellulose finish). It plays as well as ever and has chilled on the wall thru dry winters and incredibly moist summers like what we're having now, no issues.
The physics of a bass hanging by the headstock are not measurably any more hostile to the longevity of a bass's neck than those of a bass in a gig bag or hard shell case, and the main issue of humidity is that large changes can cause things like fret sprout or cracks in delicate finishes. A neck's wood giving out due to humidity could maybe be possible if the internal forces are excessive (strings tuned too high, truss rod cranked to absurdity, etc), but still unlikely without an intrinsic fault in the wood.
My guess is the original bass's neck had a weak spot in it.