It's only a matter of convincing the player base that this is so. In actuality a lot of the opposition is likely from kneejerk reactions. But you wanted to know why people might be against it. There are certainly advantages to modders getting paid for what they love to do. This isn't me being against the idea by the way.
There's already been predictions of companies springing up, pumping out cookie cutter content while over shadowing the 'true' modders. Modding will move away from being a 'labor of love' to just another business. Modders do what they do because they enjoy it, or they want to improve their games in some way. People have been modding for far longer than communities such as nexus have been around.
