There's where we disagree: I think there is a time when poetic inspiration leaves the poet, and he or she is left with calculation, experience and memory. I think the amount of revision the poet does with a poem is a good indication of whether inspiration has departed. The less revision the poet performs, the more "inspired" the poet is; the more revision he or she performs, the less inspiration there is, and the more calculation, experience and memory take over. This, of course, is just my opinion, but it comes from my own experience. Those poets who say "inspiration never leaves" are I think fooling themselves.
To make such an assertion, it would seem to me that you would need to back it up with some evidence from examples. I would agree that we have some individuals, such as Wordsworth and Coleridge who seemingly burned out or lost their inspiration. But then we have far more examples than Yeats. Goethe, Dante, Victor Hugo, Walt Whitman, Emily Dickinson, Eugenio Montale, William Blake, Pablo Neruda, Heinrich Heine, Rafael Alberti, Yves Bonnefoy, Geoffrey Hill, Octavio Paz, Chaucer, Edmund Spenser, Sir Walter Raleigh, John Donne, Robert Browning, Robert Herrick, and any number of other poets created (or are still creating) some of their most masterful works well into their life and career. The same is true of any other artistic endeavor you may wish to explore. J.S Bach's last compositions such as The Art of Fugue and A Musical Offering, Beethoven's late string quartets and 9th Symphony and Mozart's Symphonies 40 & 41, Die Zauberflöte, and Requiem all show composers at the height of their genius and inspiration. Painters? How are these for "uninspired" late offerings:
Your notion of the "inspired" work of poetry as created without effort... without revision or reworking... almost like taking dictation from some external muse is largely a Romantic fantasy. Some artists may seemingly struggle more... Beethoven immediately comes to mind... but this in no way means they are less inspired and simply calculating. Inspiration exists/occurs along the whole of the creative process.
One of my favorite quotes is by Picasso: "Inspiration exists, but it has to find you working."