I am not sure that the fact that the Martin series will be longer than projected is a stick to beat him with on its own. Tolkien himself said that his tale grew in the telling and if you look at the material briefly referred to in the appendices or worked up post publication in Unfinished tales to see that had Tolkien been a follower rather than a fore runner writing professionally for an established market with secretarial support he might have produced many many volumes of third age histoey alone. Of course you would lose the richness of the unexplored vistas though maybe new ones would have been revealed. But Tolkien was writing one novel not a series so you do get that concentration or perhaps distillation of a long process the difference perhaps between a michelin starred meal with premier cru vintage wines to a good homecooked meal.
However sometimes a writer becomes so successful that perhaps the editing process gets less robust and the work suffers. I always feel that Harry Potter and the Order of the Phoenix could have lost a couple of hundred pages but by that stage nopne dared tell JK R that it was baggy ...maybe that is what happened with the Hobbit films too.
However I cant say which is the case here until I have read Martin.... i have been tempted but I am waiting for it to be completed ... if I like it I know the waiting will drive me nuts..