Some interesting take aways from this article:

People can be motivated by intrinsic factors (eg studying hard for your exams because you really want to be a doctor and are genuinely interested in medicine) or extrinsic factors (eg studying hard for your exams because it will please your parents and help ensure that someone pays you well for your work later in life).

The overjustification effect suggests that if you reward people for something they already like doing, they’ll actually lose the motivation for doing it.[1]

So even if a modern day person would benefit from a well-timed motivational message, they’ve likely learned to ignore it due to oversaturation (my own favourite example of this was finding a motivational message on a bag of Burger King fries)

In short this article argues that they do work but because people are often overexposed to motivational quotes, posters and whatnot - the effectiveness of such mediums is completely lost and could even have the opposite effect.

1 - http://fitaba.com/page16/assets/Overjustification Study - Lepper.pdf