You will have a difficult time selling someone anything if they don’t see the value. While I agree that everyone, even myself and other fitness professionals, can get a benefit from hiring a fitness instructor (we can’t see every angle of every exercise in a mirror and we can always learn something new). Sometimes getting others to see the value requires being able to read the prospective client’s needs. I approach people from time to time with only the sincere desire to correct their form or point out a danger to the way they are exercising. And end up with a new client.
Maybe you could approach them with a bit of free advice. Or ask to show them a new exercise. Or just ask if they have changed up their routine recently to keep their improvements moving along.