If Williams does leave in August (I've heard contradictory reports), then she'll have stayed through her original contract, as the show was originally set to close on 8/31. She would be leaving early, and despite the four or so performances she missed missed for illness, she's had stellar attendance.
Everyone gets sick, and sometimes you miss a show. Even the oft-referenced Merman wasn't immune.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body