This is something I've been thinking about - in the same way some people see movie music as inherently inferior, for me opera is a trashy genre. Because the music is subservient to the story, there is no structural integration beyond recurring themes, certainly nothing comparable to sonata form or fugue. (Hindemith's Der Harmonie der Welt is an obvious exception.)
Also, most opera's plots are primitive in literary terms.
My, my, my...I'm inclined to agree with you about the music being almost an afterthought in an opera BUT Wagner has made me appreciate the genre. Bartok's Bluebeard's Castle
is another opera I love. The music behind the voices is so extraordinary that I don't pay attention to the singing, but with a movie, which, let's be honest, is completely different medium of expression altogether, I can easily focus on the story at hand and tune the music out. An opera's story has never interested me, it's the music itself that I'm fascinated by. What's interesting is people ask me why don't I just buy, for example, a recording of Wagner's preludes and overtures and don't worry with the full work. The main reason is because overtures and preludes don't contain all the music of an opera. The music that accompanies the voices is still an integral and vital part of the work.
P.S. Speaking of operas, have you ever heard Langgaard's Antikrist
? All I have to say is the music will blow you out of here. The story doesn't make any sense to me, but my goodness --- THE MUSIC!!!