And if they end up serving no practical use they end up discarded and ignored by scientists and engineers (if maybe not pure mathematicians). The author's point is that there is no such selective pressure on lit crit.
Litcrit theory is actually a pretty big commercial success. Derrida et al. sell quite well for academic books, and a Sexy Postmodernism Sex Theory Made Accessible to Undergraduates type course is a reliably decent draw for English departments, who need the enrollment.
Furthermore, if you consume entertainment products with shifting, non-linear narrative like The Simpsons, Coen Brothers movies, or Inception, you're exerting selective pressure in favor of postmodernism.