Specifically, what is a voice teacher for singers?
A teacher of singing identifies and helps to solve vocal problems. They may or may not “teach you repertoire” as part of their work, but if they are truly teaching voice, they will make singing in general easier.
Every teacher does not see the same problems at the same time. If the teacher is an effective problem-solver, they can help to solve the problems they do see. But for many singers, going to a new teacher uncovers other problems which the last teacher either did not see or did not prioritize.
Theoretically, working with a great teacher for a long time should get you to the same place as working with a few teachers for shorter times. If they all see valid problems, and help you to solve those problems, hurray! I know of a few singers who only had one teacher who sing excellently. I also know some people who have studied with many people who don’t sing well. We all know that the singer’s experience with voice training is very individual. I can’t say whether sticking with one fantastic teacher for 10 years is better than two fantastic teachers for five years each. As a singer you must walk that lonesome valley and decide whether you want to work with someone, and when.
I am not a good voice teacher because I had a great singing career. I am not good because I have “produced” a certain number of successful students. Being recognized as an authority in a niche of the voice world does not guarantee my abilities to help singers solve problems. I only can claim an ability to teach well because with every singer who comes, I see problems to solve. I can help with most of those, and with the rare problems that stump me, I try to admit where I fall short, and recommend alternatives. Other good teachers may prioritize problems that I do not. Good thing!
Now I am comfortable with saying “I am a good voice teacher.” Positive professional self-esteem can live with the fact that some people think I’m terrible, and that most people don’t care. There is no need to worry about being lauded or proving to the world that I’m hot stuff. I’m just someone with ears, training, long experience, and an ability to identify and work on problems, and I’m thankful that there are so many others who see and work differently.