If I understand correctly the sunk cost fallacy your example lacks an objectively better alternative, like an opportunity related to one of your hobbies, unrela

If I understand correctly the sunk cost fallacy your example lacks an objectively better alternative, like an opportunity related to one of your hobbies, unrelated to your studies, which could land you in a great job in some years. As you stated it, there's no doubt, staying in the PhD is the only rational choice.