It's not the election rules being rigged that benefit incumbents, it's the complacency of the average citizen. A study was done a while back that asked if a option was put on the ballot to throw everyone in Congress out, would you vote for it. Around 70% of people said yes. When asked if they would be voting for the incumbent on election day, 70% said yes.
Eric Cantor out spent his opponent 200:1 and lost. So it's not money that necessarily benefits the incumbent. Yet Harry Reed lets bills from the House sit on his desk never taking them up, plays with the slander sheild of the Senate floor to make horrid remarks of presidential candidates, and says he "wont' even look" at a bipartisan letter to reform the tax code yet in the same interview says the Republicans are causing a stale mate. He's re-elected.
There needs to be less complacency in the American people. Better primary opponents to both Democrats and Republicans.