You may be right that gravity is one of the most well-understood forces in physics and that its effects are well known, but that was never the point of Nikhil's essay. He was pointing out the absurdity of how, despite the fact that our universe is full of mysterious unknowns, many of us act as if everything is fine. The idea of gravity suddenly disappearing may seem far-fetched, but it's a great example of a potential unforeseen consequence, no matter how small the probability.
Moreover, Nikhil's essay was meant to spark contemplation about all the unknowns in our universe, since we can all agree that every answer leads to more questions. So, if we have all these theories yet so much uncertainty, how can we simply move on as if everything is fine? UrRong - it's laughable to think that we can shrug off this type of philosophical consideration with a wave of the hand.
Nikhil's essay was not meant to be taken literally - it was meant to remind us of just how much mystery there is, and to allow us to pause and think about it all, especially when life can seem so normal. At the end of the day, even if the thought of gravity suddenly disappearing is unlikely, that doesn't mean we should overlook all the other unknowns that exist in our universe. After all, stranger things have happened - who knows what's in store for us down the line?