I recall watching an interview episode featuring Warren Buffet and Bill Gates, where they were asked about the most important thing contributing to their success. Instead of answering directly, they both wrote the answer on their hand and held them up to the camera at the same time. The word "focus" was revealed and they both laughed.
I agree that focus is crucial to success. If you ask any successful person, they will explain how much focus contributed to their achievements. Kobe Bryant, one of the greatest basketball players of all time, repeatedly stated that he was not interested in anything other than basketball. He spent countless hours perfecting his craft. This attitude is admirable, but the advice to focus solely on one thing is not applicable to ordinary people like you and me.
Let's use Kobe Bryant as an example again. There are only 400 NBA players out of 7+ billion people in the world. That means the chance for an ordinary person to successfully enter the elite league is much lower than for other professions, such as doctors, lawyers, entrepreneurs, etc. Directing one's entire life into one profession or trade is like investing all your money into a single stock; the risk is high. Therefore, I would argue that a better piece of advice for young people is to be versatile.
Having multiple skills under your belt is like diversifying your portfolio; you are less susceptible to a volatile market. The skills you acquire always have synergy, combining to make you a more useful person as a whole. Imagine if you knew a bit of photography, videography, website design, and copywriting; you could jumpstart your business with zero upfront cost and reach more clients from multiple markets. It’s crucial especially in this rapid changing world.
I like the quote by Scott Adams, "Every skill you acquire doubles your odds of success." It is simple and easy to remember, and I think it applies to most people. Being a jack of all trades is a good thing, maybe even better than master of one in most occasions. As it is impossible to master everything anyway. So be versatile, as that will make you a more useful person as a whole.