indefinitely means one-time grant which is invested, and the returns on investment pay the salary of a grad student or part of a professor's salary. So yeah, it's indefinite.

These profs still have to get funds to get grad students or lab space, and still have to teach to maintain certain benefits. In later years, the endowed chair is essentially a chair in the closet as their research and teaching activity tapers off before retirement.

My spouse was an education-institution major gift fundraiser, so she dealt with endowed professorships, eternal grad student fellowhips, recurring scholarships, and the like. Generally, you can attach as many or as little strings to the gift as you want, but for most things there's no way to enforce those strings after the gift is made and the financial engine starts producing.