Well, yes you can do that for some theoretical knowledge, but it wouldn't be very practical on the job because indeed, they are not the same jobs at all. The role of nurses is to handle the expected as efficiently as possible. The job of docs is to handle anything unexpected. That is why a wide education is necessary.

You may also be surprised by the fact that specialist nurses are well paid and would often never want to "upgrade" to being a doc, precisely be cause of the additional constraints. Finally, the ego of specialist nurses rivals that of docs, and they are even stronger unionized. I don't think it's really possible to keep the same quality of care for cheaper by changing education because armies of residents and fellows are effectively doing slave labor and you'd have trouble finding cheaper and more efficient than a highly-trained slave. In my opinion, well applied technology is the only long term solution.