My understanding of the payment strategy is that it would be targeted at people who already qualify as experts but are on the fence. For example, I know postdocs and young faculty who would like to review more as a kind of community service but can't quite justify it in the face of other demands on their time. The money might be enough to tip the scales.

I don't think it will work on the other more senior pool of expert reviewers. On the other hand, I think most of the really bad reviews come from this senior pool, so maybe that's fine.