> However, being nice to passengers (performing "caring labour" as Graeber would have put it) increases the chances that people choose the bus over e.g. the metro or something else.

Increases is a relative measure. It may be 10% or it may be 0.0001%. At very small values it becomes basically a zero because humans are non-divisible unit at which point your theory is basically in the rabbit’s hole of magic and wizardry.