It's possible that poets get a little hung up on the idea of poetry sometimes. After all, a lot of the poetically-inclined literature out there deals with the difficulty of writing verse, the beauty of poetry, admiration for other poets, etc. Even in "London, 1802," a sonnet that mainly focuses upon the need for a major personality make-over of the English population, the idea of poetry shows up. Wordsworth's invocation of a great poet of the past, Milton, brings to mind the tradition of English poetry, and presents verse as a kind of guiding light for the people of Britain. Literature is subtly portrayed as a powerful and influential moral force in this poem, one that shapes the characters of the people it touches.
The corrupted moral condition of the English people as Wordsworth sees them makes it impossible for new literature to reach the same exalted standards that Milton's did.