Let's talk lucrative... Over the long term if reimbursements stay at present levels, and ER physicians practice ~10 years at the median for income for ER physicians, they will make more if they continue to work.
However, this does not take away from the fact that there is a huge nursing shortage and that's why nursing salaries are being pushed up so high. If we could flood the market with nurses (4 years of college only required), their salaries would come back from their current stratospheric levels to around 30-35K for recent graduates and an upper limit of around 60-70K for more experienced nurses.
If you look at the cycle of supply and demand, I wonder why more women (or men) do not start going into nursing as the salaries for new grads now range 40-50K, as much as newly minted computer programmers/IT people. Also, some experienced nurses who do traveling nursing or temp nursing and work per diem can make 70-100K. And you can't beat the job stability of nursing (although this is debatable, since some LP are being used as RN extenders).