In my experience, retaining instructors is a nightmare for a barn, perhaps unless they are a high end show barn that has built a program around a particular person who is a trainer of both horses and people and "brings home the ribbons." Or, a barn where the owner is also the instructor (which was the case at the last barn I was at- and she was quite honestly the best teacher of adults I've ever met, and that includes over a decade working to train school teachers). My BO only charged boarders ~$40 for a 30 minute private dressage lesson and non-boarders $50. And to teach, that meant she'd be losing time on other horse care, training, or farm maintenance, but she just loved the teaching and was truly invested in seeing partnerships between horse & rider grow. She didn't allow other instructors to teach from her farm unless it was an unusual circumstance (e.g., she approved me having trainer come do some groundwork lessons with me when I was having trailer loading issues with one of my horses).
There are a lot of thankless aspects to being an instructor. If there are kids involved, there are know-it-all parents demanding that lessons are either harder or easier. There's showing and the competitive aspect, which can make people act crazy. As an instructor, you often get roped into doing barn chores even if that's not your job ("oh, you'll be the last one to leave because your last lesson ends at 8pm, so could you just run around real quick and do night check to make sure waters are topped up and everyone has hay?"). I was on the Board of a therapeutic riding program, and also volunteered as an (unpaid) instructor, and I'd say outside of fundraising, making sure we were keeping good instructors was the hardest part of being on the Board. Instructor wages are horrible. If I recall, we paid our instructors $15/lesson, which included working with the rider to catch/tack the horse, 40 minutes mounted instruction, and then untack/groom. There were no benefits, no time off, and most lessons had to happen in the evenings or on weekends to accommodate rider schedules. To teach therapeutic riding, they had to have gone through a national certification process that costs several hundred dollars and requires several weeks of supervised training, so not a small investment on their end. They got paid the same rate for an individual lesson or a group up to 4 people. The lessons were ~$40/rider and we still lost money on every lesson when full horse care and personnel costs were factored in. Stereotyping a little here, we also generally found instructors who were young women in transitional parts of their life- e.g., had just graduated high school and were thinking about college; had small kids at home and not wanting to go back to work full time; teaching lessons part time while getting established in an entry level full time job that didn't pay very well. A lot of times when they left, it was because something changed with work or family and they no longer needed to or had the time to teach lessons. It's a thankless job; I don't really see any way to make a living as an adult on an instructor salary only.