How can a professor who has signed The "Cost of Knowledge" pledge review papers without having to recant? A large majority of researchers in my field publish in Elsevier journals. However, a professor who is widely regarded as an authority in my field has, some years ago, signed The Cost of Knowledge pledge to neither publish nor review for Elsevier. I think this is sad, since someone who is highly qualified to review these papers will no longer review them and improve their quality. 

This professor has published in other journals, but it does not seem that others are following suit, since these Elsevier journals seem to be highly prized. What can he do to be able to review papers that would have otherwise come to his perusal without having to recant?
Sounds like he's sticking to his moral principles, which is the best anyone can do. Maybe he could evangelize his opinion more to increase the impact of his crusade, but that's about it.

I disagree with the insinuation that he may be doing his field a disservice, since moving away from the ancient publishing model is a good thing for any field in the long run. Besides, it's not like he has left the field; just a small part of its publication scene.

Just because nobody seems to be following him (yet) does not mean he failed in any way.