… admitting that there may be a subjective element to literary critical judgments does not entail that all such judgments are subjective, or entirely subjective. Literary judgments may be objective up to a point, but not beyond that point. The precise point where objectivity runs out would depend on the work in question. Accepting that there may be subjective patches in the field of literary criticism does not entail an acceptance that the whole endeavour is subjective and (therefore) pointless.
Obviously, if only science or mathematics can arrive at the truth, then there's no reason for putting anyone on trial for a crime, since guilt or innocence cannot really be determined scientifically in any strict sense. Regarding literature, there is certainly objectively bad writing. A plot can be demonstrably nonsensical (I give you The Da Vinci Code). It is in fact easy to demonstrate why a bad book is bad. But in some cases, not everyone will agree. I think Cormac McCarthy's The Road is objectively bad. Lots of people do not share my view. I think they are wrong. They think I am.