The laundering will work the opposite way. Instead of hiding behind one big entity, political advertiser's will hide behind a multitude of smaller entities to make themselves look like grassroots organizations.
You won't see "promoted by [Koch|Soros], who spent $8.5m on ads this year", you'll see things promoted by "milualkee citizens for freedom, who spent $1500 this year". But the money will be coming from the same place.
If you give advertisers more space, they'll figure out a way to use it effectively. Even if the point of that space was to try and keep them honest.