Doesn't my choice of major now determine what I'll be doing for the next 20-30 years?
Probably not. As noted above, there are some career fields that do require a specific major. These tend to be primarily in the engineering fields, health care, education, and some areas of business. But there are many careers for which companies are willing to hire and train new college graduates. Additionally, the longer you are in the job market, the less important your major becomes. The skills you acquire as you are working will be far more important in determining your career path. In today's job market you can expect to have several career changes. You will want your college education to give you a strong base on which to build the skills which will enable you to continue to grow and learn after graduation.