In my experience it is taken as a given that people will continue needing medical care of one sort or another, so doctors at the microlevel don't worry too much about patients getting _too well_. The exception to this would be some corners of medical care with a lot of discretionary spending like cosmetic surgery, dentistry, etc. and even there a doctor doing work which doesn't need to be done is not looked upon favorably.

Given that there is no magic cure everything potion, I highly doubt doctors will ever have an issue with a lack of patients. This is especially true if you consider population growth.