If an advertiser is dissatisfied, he withdraws his ads and is replaced. If noone wants to advertise on controversial topics, wikipedia has more than enough uncontroversial content to pay the bills. They might even just disable ads on controversial pages, which would go a long way towards avoiding even percieved conflicts of interest.

I don't think anything would happen.

They should just get one big sponsor per year to cover their costs and be done with it. Why not have a McDonalds or Pepsi banner somewhere in the corner? Bargain branding for them, free wikipedia for us.

And to limit the influence they could make it a rule to switch sponsor once a year. Why not auction the ad-spot and donate the overflow to charity?

or just keep the extra as an endowment and eventually just run off that

What happens when Google says they won't donate to Wikipedia unless the Google article represents them more favorably?