Classical Roman architecture is beautiful, primarily because of its use of rounded arches and its symmetry. Postmodern architecture is dramatic, primarily because of its creative use both of materials and of the surrounding environment. An architectural style that combines elements of both classical Roman and postmodern architecture would ttherefore be both beautiful and dramatic.
The reasoning in the argument is flawed because it presumes, without providing justification, that for an architectural style to have certain qualities, its components must have those qualities.