I've been an instructor and coach for 20+ years and will say this: Tennis instruction is a terrific part time job, but I would not want to rely on it as my sole source of income for a variety of reasons, eg, little job security, poor pay, no benefits, fragmented scheduling and the trials and vicissitudes of dealing with the public.
On the balance though, I've loved almost all of it, from being an assistant on a nationally ranked college team, to working weekends at an adult camp to a few gigs at a high-end resort to coaching aspiring juniors to teaching locals at the public park at noontime. I like chatting with my players when we pick up balls, sweeping the courts and planning my clinics. Last week, I coached a women's 4.5 team for a 2 hour clinic then went directly (ie to the next court) to "coaching" a couple of 4 year olds (the latter of which was a blast but very draining). I've had remarkable experiences, have met some terrific people and get to be outdoors. However, having witnessed the pressures other instructors/coaches have endured as they try to make a living in a field saturated with wanna-be instructors, facilities on the verge of financial collapse and employers who often look at you as just a fungible part in the sputtering machine, I'm glad I had a career in business that enabled me to make enough money so that I could enjoy coaching tennis without worry about compensation, etc.
