This comes from David Perkins's A History of Modern Poetry: From the 1890s to the High Modernist Mode (1976; pgs 128-130; it is the first of a two volume history), closing off his discussion of Edward Arlington Robinson. The ideas he raises are both historical and theoretical in nature.
The passage may be thought to be weaked by Perkins's not maintaining throughout his discerning between genre literature and aesthetic literature, to which he points directly in the moment about Don Juan and Four Quartets, and which is the true center of the argument. When Perkins reaches "the audience for long poems no longer exists," what he is saying is that the historical genres of long-form verse no longer exist (thus, no readers of those genres). Though, that may be intentional, as that recognition opens the door to a discussion that leads this short excursion into long explorations.