One man cannot practice many arts with success.

Mastery is expensive in a way that does not divide well. The hours required to be genuinely good at something are large enough that spreading them across four pursuits usually produces four competent-looking efforts and no depth anywhere. This is less a law about human capacity than a fact about arithmetic and attention.

That said, seasons exist, and what you concentrate on can change over a life. The question is what you are choosing for this stretch, not forever. If you are currently mediocre at several things you care about, picking one for the next year is likely to change more than trying harder at all of them.