At this point your argument seems to be "we cannot come up with a rigid ordinal ranking system, therefore no gradiations of value are possible" which is a little like saying that because we cannot catch all murderers, we should not try to catch any.
The real answer is that we do what we can. Earthsea is better than Eragon, and that will probably meet just about every informed opinion. Therefore, we can make assessments. I can probably name a hundred fantasy genre books that informed consensus will agree is better than Eragon, and very few published works that informed consesnus will agree rank lower than eragon on the quality scale. More fine tuning is not necessary. Does it really matter if Eragon is better or worse than The Younger Gods or Left Behind? They are all lousy writing, and all of no real value as books.
We don't have to rank books in rigid order. This isn't some E! channel list of "100 Greatest Fashion Gaffes!" or something like that. We just have to evaluate them well enough to tell what are good works, and what are not. And that we can do.