They might lose revenue at first, but in a way that is entirely healthy for a larger system of advertisers and clients, in that both advertiser and client reassess the effectiveness of an instrument (an ad network or product), and thus reassess the correctness of a price for an advertising opportunity.

Maybe I'm apathetic to these arguments because I'm quite comfortable paying for content and services, and I'm comfortable with a version of the web where a paying class gets to have paying-class products. If the public feels there should be a free thing, then let it be funded by taxes.