Okay, let's think.
For the quote to have become well known, it must have been amusing or ironic.
So the opera to which it referred may have been something quite special.
I'm thinking ... The Ring Cycle, perhaps?
And the opear that Wagner wrote after the Ring, was ...
Okay, Tristan and Maestersinger were written while the Ring was still a work in progress.
I think the only opera written AFTER the Ring was Parsifal.
So, this may be totally on the wrong track, but my guess is ... PARSIFAL.