I don't think it's a question of preventing political corruption. I think Rowling wanted to portray it because it is a prevalent part of our lives today. She is very good about using everyday lives and past history to illustrate the points she drives home in the HP novels.

As for the political corruption in the books, it can't be avoided. Even if one or two people knew exactly what Fudge was doing, and managed to overthrow him, "Corrupt Power Corrupts Absolutely." It is a fact that any time you have people in charge of a government or in a position of power, they use that power to their advantage, even if they don't mean to--and they start out intending to use that power justly. People are always corrupt. It is just the way we are.

Think about Communism. In theory, it is a great idea. It doesn't work at all because people are involved. Any time you get people involved, the idea goes awry.