Also, rigidity isn't a bad thing. If they mean that it's limiting, they'd still be wrong: it's not. When a particular rigid thing is limiting, it means that that particular rigid thing is not generally applicable. The case with structuralism is that it is a rigid thing that is generally applicable. It's not limiting in any way. Certainly, if you are considering a particular set of relations, you're limiting yourself to that set of relations (and the (sub-) structure that they generate), but that doesn't mean you can't, when you've temporarily stopped considering those relations, consider other relations. The rigidity, contrariwise, is helpful, because it keeps different substructures from confusing.