I see your point & agree with it to an extent. But it is one thing to use a heuristic model or a metaphor to emphasize a structural problem that may not be visible to a naked eye, but quite a different one to attack that problem with specific demands and counterproposals.
To say it differently - suppose that you manage to convince a sufficient majority that capitalism sucks. "OK" they would say "but what do you propose instead? That is to say, what specific social-economic-political institutions do you propose instead of the existing ones?"
In a way, the problem is inherited from Karl MArx who was quite convincing in identifying and problems in capitalism, but rather vague in presenting solutions.
The focus on corporate behavior at least keeps social change attainable in the near future. And it does not have to be grounded in market ideology - corporations can also be a tool of economic planning and overcoming ineffciency of the market (the so-called transaction costs cf. Oliver Williamson). Stated differently, existing corporations are bad not because they subvert markets, but because they subvert their potential for rational planning imbedded in their structure (which, I believe, was also the point raised by Galbraith).