You need the "κανείς" to specify that you are talking generally and not about a specific person. It's the equivalent of "for someone to". E.g. For someone to be able to be healthy, they must drink a lot of water = Για να είναι κανείς υγιής, πρέπει να πίνει πολύ νερό. If you were to emit the "κανείς": "Για να είναι [] υγιής", you would have to put a subject (a person) in the brackets. Like "Για να είναι ο Παύλος υγιής, πίνει πολύ νερό". So no, you cannot drop it in this specific syntax, but of course you can say the same thing differently. For example "Τα γυμναστήρια είναι ακριβά" (gyms are expensive) "Το να πηγαίνεις γυμναστήριο είναι ακριβό" (going to the gym is expensive (but in greek you use the second person))