This article makes some good points. The book it cites sounds interesting. But it also lacks some nuance:
Whether you need to specialise early to be a success varies greatly by field. It's hard to become a world-class violinist or gymnast unless you do serious training from a very early age. The same is NOT true for, e.g., a law professor or a scientist.
Whether "grit" succeeds in making you a success in a given activity or field depends in significant part on whether you like the activity and have an affinity for it. A person who dislikes playing the violin (as I did when I took lessons) is unlikely to become a successful violinist (even if, unlike me, they have the talent for it).
It is indeed important to know when to quit, but also important to know not to quit too early or at the first sign of resistance. Whether quitting makes sense depends on a variety of factors (including what else you can be doing with your time). It's not always easy to say when "don't be a quitter" is better advice than "don't reinforce failure."