When you talk to a bike messenger you will hear a few recurring themes.

--Pay is terrible
--VERY few full time jobs left
--Pay is getting worse
--Number of jobs is still declining

Email has killed it and in my opinion will keep killing it. When you saw the glamour era of bike messengers it was in the 80's before the web. Far be it from me to keep you from following your dream but people who end up being successful and comfortable in life tend not to enter professions in which you can see obsolescence in the near future.