Retrospectively ascribing success to character traits that we favour is subjective to the point of meaninglessness.

Earnestness, like any other trait or 'virtue' is not innate or absolute. Everyone has the capacity to exhibit this trait or be seen to exhibit it, when refracted through the non-objective lens of someone else's bias and perception. For example, a politician for partyX is seen to be an earnest 'straight man' who cares deeply by his supporters, while supporters of partyY view him as a reckless oaf.

To promote the use of personality traits as a predictor of success is naive at best.