Another angle is that sometimes we also should be investing into ourselves in ways that would potentially cause us to work smarter rather than working harder and also we might be able to increase our income by being able to get higher paying jobs by investing into ourselves (such as training, education and/or getting experiences that might not pay well in early years, but have greater potential to pay off higher at later times down the road).
Updating oneself or improving oneself is a sign of a wise person. A wise person always strives to improve himself and must invest in it. Investment here does not mean that you have to invest money, sometimes you have to invest time to improve your skills, sometimes you have to invest labor, investing time can improve your skills and sometimes you have to invest labor, time and money. have to do Investing in yourself never hurts when it comes to investing, of course your knowledge will increase, it's a return on investment and your salary is likely to increase or your earnings will increase as a bonus for you. So always try to update yourself and invest some time in it. Investing in yourself can help your portfolio grow faster.
Yea you are right, and I also the same sentiment as you, because I am quite aware that the best investment anyone can ever make in this life is by investing in himself.
So by investing in your education, your growth, your mental health and your health is a perfect template for an individual to be able to grow and face the world, because knowledge is very much key in every sector of life, and their is a saying in my country that goes like this;
If you think that education is too expensive, try ignorance. So in essence of what am trying to say is that, without updating and improving yourself in this evolving and developing world, you are bound to hit a rock at some point in the future, but if you are educated enough in all aspects, I believe that your decision making will speak volume for you.