Wimsatt and Beardsley's essay The Intentional Fallacy wasn't flogging a dead horse, nor did it bury the concept of authorial intent.
One of the most influential statements of intentionalism is E. D. Hirsch's book Validity in Interpretation (1967). In an essay entitled "Why Intentionalism Won’t Go Away", Denis Dutton describes Hirsch's stance as follows (my emphasis):
Hirsch’s intentionalism stands apart from that of someone like Tolstoy because it is not so much a particular conception of art which motivates him to adopt it as it is a strongly held view of criticism. For Hirsch, unless we have a standard of interpretive correctness, criticism loses its status as a cognitive discipline. Without a notion of the author’s meaning as a guide — almost a regulative ideal, it would seem — criticism would be unable to decide between competing interpretations of works of literature (or art). The result, for Hirsch, would be chaos: anybody’s interpretation as good as anybody else’s. Hirsch does not deny, of course, that works of art may mean different things to critics or to audiences in different historical epochs. This is in fact how it is that works of art can have different significances to people. But the meaning of a text is always one and the same thing: it is a meaning that the work had for its maker, the artist or writer.
Hirsch's brand of intentionalism isn't the only one; there is also a weaker form known as hypothetical intentionalism. One representative of this type of intentionalism, Alexander Nehamas, argues that
interpretation is a matter of attributing an intended meaning to a hypothetical author, distinct from the historical writer. This view allows the interpreter to find meaning even in features of the work that may have been mere accidents on the part of the historical writer.
(Quoted from Teaching and Learning Guide for: Authors, Intentions and Literary Meaning by Sherri Irvin.)
In summary, one can say that intentionalism is not dead but is the subject of theoretical debate.
