What to call a doctor who doesn't really cure their patient with their effort (to get more money from "repeat business")?

Shyster

a slang word for someone who acts in a disreputable, unethical, or unscrupulous way, especially in the practice of law, sometimes also politics or business.

http://wikipedia.org/wiki/Shyster

If you LITERALLY mean the doctor (or perhaps, car mechanic) DELIBERATELY does not cure the patient, so that the patient will come back - so, exactly as in the joke stated.

The only thing I can really think of there is that the doctor would draw out the treatment, or perhaps someone like a car mechanic would string along a customer. In both cases they are milking it for all it is worth.

General terms like mercenary, bastard, charlatan, etc could mean many things. None of those specifically mean anything like "deliberately giving a lesser treatment than would be ideal, so as to extend the customer's payments over time".

The only phrases I can think of are the three mentioned above.

(It makes me think of Ralph Nader and planned obsolescence. Perhaps Nader had a word for this?! Like "under-servicing" perhaps.)

A sham doctor is called a "quack."

A very profit-driven quack is sometimes called a "snake-oil salesman," after men in the 1800s (roughly) who travelled around (often with a small circus, or "medicine show") peddling phony miracle cures allegedly made from ingredients like snake oil. That's probably the wrong connotation, though, since a snake-oil salesman didn't string along a small number of customers for a long time; he just sold as much snake oil as he could and then skedaddled before they caught on.