You can always transfer money from point A to point B, the only difference is how costly and transparent that transaction is is. Banning private contributions increases the cost but decreases the transparency.

There's a significant competitive advantage to writing your own rules for the game you're playing in, so as long as that much power is concentrated in so few hands, businesses are going to find loopholes and other means of getting what they want. An example would be a business threatening to fire all their workers in a certain area and move somewhere else unless they were granted what they wanted. Boeing did that quite effectively just recently in the state of Washington and was able to get a massive, multi-billion dollar tax break. So now Washington taxpayers are essentially subsidizing the bottom line of an already hugely profitable business.