He'll have served his purpose by the end of 2016. While I wouldn't mind having him for 2017, I'm sure we can find a much younger and more long-term DP solution at that time.
Or Ibra.
Random daydreaming: I wish we could ax Lampard after this season, and sign up Ibra when his expires next June. Pirlo feeding Villa and Ibra? Holy fuck.
I guess the keyword is option. So if he's kicking ass here, why not have the ability to pick it up. If not, then we move along. Unless the option is exclusively his.