Or so New York's Metropolitan Opera seems to think:
Shown: Image by Suby Raman. Not shown: Operas by Kaija Saariaho, Francesca Caccini, Judith Weir.
Also, the median year of composition for a Met production (1870) hasn't changed in over a century. Put another way, an opera produced in 1905 was on average 35 years old, while an opera produced by the Met today is on average nearly 150 years old.
Thanks to Suby Raman for doing this research! For more fascinating statistics on the Met's, er, backwardness, read here.