the.com/linchpin
The small thing whose absence makes everything else fall off the wagon.
means The single critical part, person, or idea that holds an entire system together.
from Literally the pin slotted through a wagon axle to keep the wheel from rolling away — pull it, and the cart collapses; from Old English lynis, axle-pin.
Spelling fightLinchpin and lynchpin are both accepted variants.
Not that lynchUnrelated to Lynch the man; pure axle origin.
Still literalReal linchpins still secure trailer wheels today.