He attracted students from all over the Greek speaking world, many of whom went on to important leaders of their day. Isocrates died at the age of 98. According to tradition he committed suicide by starving himself to death upon hearing of Philip's conquest at the Battle of Charonea. This seems unlikely, since he encouraged Philip to do this. He thought the tedious, abstract arguments of Plato about metaphysics, epistemology, axiology and human nature were so much jibberish.
Metaphysical speculation is a waste of time and energy.
Knowledge is tentative. We can't know anything for sure. What we can have is good opinions. A good opinion is one that helps explain life in a way that helps me get along in the world. Correspondence to ultimate reality is not important.