Nothing says you have to be poor to write musicals. I, for one, do my best work when my stomach is not growling. So you will need a steady source of income while pursuing your theatrical dreams. If you have generous parents, a large inheritance or a well-heeled spouse, then you are a lucky dog and can skip to the next section. However, if you are not so lucky, you will need a job that keeps the bills paid.
Some people have a truly profitable career and write on the side.
This looks idyllic in Rent , but it soon palls as a lifestyle. If you care about the theatre enough to write for it, any theatre job is bound to become an emotional drain. In fact, I don't know of anyone who has written anything of consequence while working in a theatrical office. So seek your paycheck in a separate field. If you are going to write for the musical theatre, you have to be realistic about where the business is today.