If you have a career as a performer for 20, 30 or even 40 years, you gain experience that cannot be gained in any other way. You live singing, every day, you are a singer and you deal with whatever music you do, but you deal with traveling, with other performers, with musicians, with your own voice and health. You face multitudes of things that teach you about yourself and your own experiences singing. VERY valuable indeed.
If, then, you “retire” from singing (no real singer ever totally retires, if you ask me), and go into teaching, you may have only your own rich wonderful but very personal experiences to use as a basis for your teaching. This might serve you well in very general ways. If you have students who are similar to you in voice and interests, if you have students who will sing the same repertoire you sang, if you have to teach them to perform in repertoire you did, then it could work brilliantly.
If, however, your students are very different from you, if they have different voices, different bodies, different interests and requirements, you could be really lost. That might actually not be so bad, if you had a sense of your own limitations and could say that you were lost. If, however, you decided to hide from that fact (even to yourself) or camouflage it from others, you could be in big trouble. If you decided that you would plunge headlong into teaching anyway, regardless of what you have to offer and its applicability to others, you could be traveling down the proverbial slippery slope. If you take your student somewhere the student has no ability or desire to go, but has to go anyway since the person is after all, a student, you are highly likely to cause some kind of damage.
The advantage that a career teacher has is that the teacher has long decades of teaching study and experience. It allows the person to study pedagogy in depth and to acquire skills geared exclusively at teaching rather than just at singing. In other words, a career teacher knows many pathways to a destination, not just one. Someone who has decades of teaching under her belt knows what people do when. She has had time to observe trends, tendencies, pitfalls, obstacles, as well as short cuts, direct paths, and simple solutions. Assuming the teacher has also sung, and continued to sing, throughout her teaching career (an important ingredient), knowing what to do for each student, individually, in whatever shape or form the person and the voice may be in at any given time, is a skill that takes a long time to develop. If the teacher has also gone to all kinds of performances, to all kinds of venues and seen all kinds of singers, and has used the breadth of that exposure to inform the teaching, then the information gained adds to her toolbox.
In other words, performing does not mean you are a good teacher, even if you are a very excellent vocalist. All you have to do is watch American Idol and you will see that the performers who have become “judges” (if you will excuse the word), often don’t have a clue as to what to say to help a student other than platitudes or suggestions aimed at getting them to stop doing something they don’t like.
Having judged a number of classical singing competitions in my day I know for a fact that opera singers often have very strange ideas about what constitutes good singing and good singers.
In the end, being able to understand a complex process, which singing is, takes a lot. Many things have to come together in one person, at one time, and the ability to communicate from that base has to be accurate, precise, practical and clear. Is it any wonder, then, that there are so few actual masters of the process, even amongst those who profess to be?