There's some evidence his success came when he collaborated with the Expanse author. When he left Martin never achieved the same output or quality, and has merely been coasting on his initial success.

Game of Thrones is a legitimately good tragedy. However Martin has never referred to it as such which makes me believe it was an accident, and he has no concept of the literary theories he treads on. Deconstruction is fine but you have to stick the landing (e.g. the works of Avellone). Martin has ultimately nothing to say other than bitterness and spite.