Candidates who are trying to unseat an incumbent are at a huge disadvantage unless they have the type of bankroll that can even the playing field. In most cases, the longer one is in office, the greater the chance that they can put together a war chest that can hold off most challengers.
On a related note, a topic came up at a Candidates Forum I attended Wednesday night was term limits. This is something I'm 100 % in favor of, regarding the United States. Terms limits would help even the playing field in many Congressional, State, and Local races.
There are some politicians who can make a positive difference if given the opportunity. The problem is most of them don't have the funds that are necessary to win a race. Especially against a long time incumbent.