Why don’t all doctors start out as Nurses and then get additional responsibilities as they gain experience finally moving up into the realm of surgeon or whatever other specialty. I could see qualifying nurses in CFR and greatly reducing the need for full doctors. (CFR- Cold, Flu, Runny nose. A nurse could easily address basic sicknesses and refer unusual symptoms to a doctor.)
Nurses and doctors actually do quite different jobs. A nurse is not a cheap doctor.
However, there is actually a lot of the idea you are talking about being implemented. So, for example, one now sees physicians assistants, who train in particular things and can do certain jobs that doctors then do not have to do. Or nurse practitioners, who can meet many patient needs as well as a doctor (my husband had a nurse practitioner as his assigned “doctor” when he was in the military, and she was really great.) Considering the best way to set up a medical practice is another way to look at it. In some, the patient may well first see a nurse who will handle things like blood work, taking vitals, things like diabetes or sexual health education - all of which many nurses are better at.
As well, in many places all medical care is co-ordinated through a GP. If a patient wants to see a specialist, they get a referral. This solves a lot of difficulties with patients who misdiagnose their problems, use a more expensive doctor when a GP would work as well or better, and also means that someone is looking at the big picture of the patient’s health (and ideally of the whole family.)
Doctors are not always very keen on these kinds of initiatives, I think because of money, and also power. Even in places with socialized medicine they often resist, but in such cases they can be made to change to some extent.
Where I live, there has recently been resistance to the introduction of nurse practitioners and midwives, despite evidence that both result in better care and more economical care. Some doctors actually refuse to work with them, or try to set up a system where they have so little leeway that the benefits are largely lost.