Completely untrue. You don't have to replace them, you just have to bully them into submission, if you threaten their reelection then you can do that. In fact a lot of times it's easier than that, get a few hundred people calling their office, or give them bad press, and they tend to change their tune most of the time.
As an activist you must be able to mobilize people to put pressure on the politicians. Other than writing big checks, that's the only way things change.
See this for a better understanding:
http://training4liberty.org/facl2/info.htm#