Yet Vendler wants to see more than this--wants, in fact, to hear the poem's four concluding lines as being less than the "sixain" pattern she believes to be in some way expected.
Yeats ends his poem on a quatrain rather than a "sixain" precisely to voice his utterly conclusive intention: the poem's last four lines must be a defiant stance, one that develops from the rest of the poem but is shaped differently and (in terms of proximity of rhymes) more solidly.
The sixain aabccb, with every line a regular decasyllable, provides this excellently; and as it is not a common form, it mixes the requisite strangeness with its sobriety.