EDIT: And I guess I disagree with your main point anyway. Criticising a generalised attack on billionaires in terms of the effectiveness of its messaging is a different argument to saying it is inherently unfair to attack billionaires.
A more specific attack can say that the system we have is currently unfair, and allows people like this *points to bad people* to become ridiculously wealthy from immoral practices and allows them to contribute nothing to society. Just saying "Being a billionaire is bad" can be too easily twisted into the old attack line of Labour policy being rooted in wealth envy and accusations of creating equality by dragging down.
Synecdoche can be a very useful tool in politics, imo; specific examples of problems, their effects and their causes, can dig into people's brains more easily than generalisations, and are often more difficult to attack. I personally believe that "Labour want to deal with people like Mike Ashley and Philip Green" is a more effective message to put to people than "Labour think billionaires are bad".
