Indeed, to the degree that the Culture represents an aspirational fantasy, it is not "what if society were dominated and controlled by vastly powerful machines"

Indeed, to the degree that the Culture represents an aspirational fantasy, it is not "what if society were dominated and controlled by vastly powerful machines", because that is the world we already live in, but "wouldn't it be nice if the vastly powerful machines which dominate our lives liked us and cared about our well-being?"