I did not see this one coming, and from the flurry of tweets I'm seeing, it seems nobody else did. Michael Cooper has the story, above the fold at the digital NY Times. Short version: the renovation of Avery Fisher (sic) Hall will not start before 2019 and won't be completed before 2021. That is longer than Gilbert had intended to stay, and he feels it's best for a new music director to have time to establish rapport with the audience before the renovation starts. Implicit in this is that he would have been leaving somewhere between 2017 and 2021 in any event.
You've got to hand it to him for thinking ahead, and very seriously, about this. The Board has a lot of time to establish a search committee, thanks to the early notice, and make a decision before Gilbert leaves.
I'm sorry to see him go. He's done a world of good for the orchestra's programming, between the residencies, the commissions, the new music biennial, the semi-staged operas, and, well, what they've played on a day to day basis. I have only heard them under him one, but oh boy it was a memorable concert, at Carnegie Hall. I admire his foresight in thinking through the orchestra's needs and his own in such a clearminded fashion.