Not everyone is convinced that American opera has been on its last legs for a while, and so those who are doubtful as to the marginal role opera plays in American musical and theatrical culture might imagine that John Adams' Doctor Atomic could be some kind of substantial achievement. Maybe it is, that's not exactly Teachout's point, which is that for American opera to even be dying it has to have ever been culturally relevant to begin with.
Serious opera or highbrow opera has never been all that germane to American musical life, whereas light opera, operetta and even more prominently the musical (Broadway and other) has been prominent in American music. Early jazz can be thought of as a mixture of blues, ratime (and its offshoots) and the American songbook, which is in many ways another way of describing musical theater traditions ranging from Broadway to minstrelsy if we go further back.
None of that fits what would be considered the more "highbrow" musical traditions of opera, whether buffa or seria.
On the whole I think Teachout's observation that opera has been too marginal in American musical culture to even be "dead" seems basically persuasive. I'm not suggesting people can't dissent from that observation, I just take the observation for granted. American music culture has such a vast thriving body of popular song and eevn art song if we can't find a better term for that music that opera in the conventionally recognized sense of the term isn't a significant part ... unless we maybe talked about rock opera.