I believe early retirement is great for people who don't enjoy their work. For them, working a lot when they were young, and get enough money to spend after retirement, and retire early looks great. From then, they definetely can enjoy their life!
Of course, you can feel yourself as useless person after retirement. No one will find you, and no more income. However, you should realize that you are meaningful person, even though no one finds you. The most important thing is just enjoying your life! Ourselves are all meaningful people. The interests and concerns of other people isn't in proportion with how meaningful person you are.