My insurers are refusing to replace two car.

Yes, that's where rust is likely to form, ... Just like on your other two doors. Replacing just the outer sheet metal is the normal way of repairing that kind of damage and it's not realistic to expect your insurance company to pay for anything different. Don't try to bully the insurance company into doing more than is required based on the word of a "car buff". I'm a car buff too, and I know how insurance companies work and why they often get an undeserved bad rap. If the repairs are done properly, the seams won't rust any sooner than if the entire doors were replaced.

By leaving the main, undamaged part of the doors on the car, the mechanic will have a much easier time making the new panels line up and fit correctly with the right gaps and adjustments. Replacement full doors may never be able to be made to fit perfectly no matter how much time is spent adjusting and tweaking them. Even in the unlikely event you convince the insurance company to pay for complete doors, the additional paint, the labor times to install and adjust them and transfer all the glass and hardware, you will pay for it in the end with higher premiums. Your premiums are based on the risk they're taking and your insurance history. Both of those justify charging you more in the future. They're offering the proper repair. Take it and be grateful.