No one has ideal preparation, but it seems to me that a person who really struggled to sing well, who worked hard and became a fine singer, would have an ideal background for teaching, in the domain of singing experience. Then there is the knowledge side, which can be acquired by study of the bookish sort. Then there is the desire to teach. Where there is a will there is a way, and a teacher with all three of these things is bound to do rather well. Other qualifications would be icing and sprinkles on the cake.
This means that a teacher can continue to teach well, even after losing some or all of their vocal ability, if they understand the process of developing a voice from the inside out. No amount of proximity or osmosis or booklearning can substitute for the teacher who has acquired their vocal technique through hard work on their own voice. Ideally a teacher continues to work on their voice throughout their career, if at all possible.
It also means that if circumstances did not allow the teacher to have a major performing career, they may have much to offer anyway. Freeing and developing a voice does not necessarily include the fine points of verismo performance practice, how to audition for the Met competition, or how to work the stage as a lead singer for a rock band.
As the years have gone by, I have become increasingly frank about my own limitations, but also more clear about what value I can offer. I have taught people who sing better than me how to improve. I have helped singers in genres I may never perform to understand their voices better. I say to the blues band lead singer “I don’t know your music well, but I do know your voice, and I want to help if I can.” And so he comes to lessons, and there is much good work done in making the voice strong, flexible, and responsive, so that he can take it to wherever his music leads.
People generally come to my studio to solve a problem of some kind, not to look for someone to emulate. I’m a facilitator through the process of vocal self-discovery, not Brian-discovery. I try to be bold, respectful, honest, curious, and resourceful. I’m not a perfect teacher, singer, or role model for every student. Thankfully, that is OK, but I work at it!