I particularly like the argument that older beginners cannot "really" improve, perhaps small increments, but really not worth anyone's time.
Only the serve. All other strokes can be continuously improved by yourself. That is how my groundies reached the level they are today. It is true that they too plateau, but the plateau is based on fitness and age. No one is hitting a 100 mph forehand once a year and 40 mph ones rest of the time.
The serve plateau should be considerably higher for most people but isn't. Players are hitting one ace with one bounce to the fence once a year - so they can do that. But the rest of the time, it doesn't happen. So it means the ingredients for improvement are there. But no coach can tease it out. That is why most older players regard lessons to be a waste of money. I don't want your warm-up feeds and artificial drills with made-up rules I can't follow and I don't want to pick up the balls. And then you change the rules to another set of made-up rules - I will hit there and then you return here and after 2 exchanges, you can hit where you want. No - I would just like to follow one set of rules which is called tennis. And then of course nothing whatsoever to add to the serve, except hit up and out, place your toss and don't chuck it, serve is like a throwing motion, etc. Hilarious thing is when the student is made to serve and it is a fault - coach says Great keep going. Works for juniors who will eventually learn the technique AND get the serve in. But not for oldies. The correct response should be: Serve is a fault. Is a fault. Is a fault. Again a fault. You are a fool to hit up on the ball and not get it in. Each serve has to be in, fatso - didn't you study the rules? But no it is always "Great job - add more spin next time" And then of course in an actual match, the serve is a fault, and the lessons have actually made the player worse.
Reminds me of the nasty Dad I saw last weekend coaching his son. Gave him a basket of balls, and took the returner's position and started counting. Each time the ball went into the net or was out, count was reset to 0. Poor guy did not make it past the count of 3. That is the kind of tough love that adults really need. But since they don't care for it and the coach is there for the repeat money, why bother with it at all?