People can have different opinions on how harsh the criticism 'by all rights' should be. And in the case of series of books (or movies, or other types of creative work), a weaker, or in this case not-yet-existing ending reflects poorly on the series as a whole, but the individual books / works at the start may still remain very good. And I'd still rate Martin higher as a writer than other fantasy writers who did manage to finish their series without an obvious drop in quality along the way, but never produced a single book of the quality of aGoT, aCoK and aSoS.