Every fund manager holding clients' money is a fiduciary. And all fund managers get paid by the client. Some get a percentage, some get a fixed fee. The percentage is an incentive to some. Some do a good job and some not so good. It would have been more accurate for you to say a fiduciary (any broker or money manager) is obligated to try to get a high return for the client. Perfection is neither possible nor required.
What is a Fiduciary?
A fiduciary is a person who acts on behalf of another person, or persons to manage assets. Essentially, a fiduciary is a person or organization that owes to another the duties of good faith and trust. The highest legal duty of one party to another, it also involves being bound ethically to act in the other's best interests. A fiduciary might be responsible for general well-being, but often it involves finances — managing the assets of another person, or of a group of people, for example. Money managers, financial advisors, bankers, accountants, executors, board members, and corporate officers can all be considered fiduciaries.