Philosophy is the study of everything that is not well understood or is difficult to understand, even the study of study itself. A good way to think of it is, "philosophy exists only in the gaps", the gaps in our understanding. What is the nature of matter, can you take clay and keep dividing it in half and in half? When we didn't know the answer, only speculated about it, it was philosophy. Now that we know the answer, it's physics. But unknown answers in physics are still considered philosophy, such as whether or what would it mean if time can run backward.

This implies philosophy and philosophers will disappear once we know everything, which could by some be considered one of the rewards of this quest for truth.

BTW, many institutions today offer the ScD degree, doctor of science; it is generally equivalent to a PhD, though in some places it can be considered "higher"