Reverse that SC decision, you harm speech like Michael Moore's.
To your other point, there would be no corporate sponsors if we didn't concentrate the government's power in Washington by forgetting we can solve local problems using local government, and if Washington actually let businesses compete fairly and not creating artificial barriers to entry, such as license fees, over-burdensome regulation, frivolous patents and high taxes.
The problem isn't just corporations, which BTW, except for monopolies, are a net benefit to society (otherwise they just disappear). The problem is that we vote for corrupt politicians who corrupt our laws and interfere with the market killing off bad corporations.
James Madison said "If men were angels, no government would be necessary." The corollary is that government is not comprised of angels, therefore we can't expect it to be necessarily better than corporations.
I do agree that not voting in corrupt people (and really punishing those who are shown to be corrupt; not just reading a letter in Congress). More importantly, move the power and influence closer to the people. I have far more sway over my mayor than I do over the President, and I'd trust him with more of my money.
That seems backwards. If reversing that SC decision would harm speech like Michael Moore's then it would make sense to say that without "Citizens United", there would be no Michael Moore.