Their love of music will always continue
This year Thurston was saying he was hoping they could turn their sound totally around. by being able to go any amount of time without having to feel like sonic youth exists, the sonic youth "idea" or "mystery" will be resting fully underground like a cicada and could one day emerge as a new music entity.
2015 might be a little early. but i take Lee's mentioning of Radical Adults and the idea of "elder" humans making some of the most pure abstract and concrete statements around that something will happen someday even if it is just a new song. when it happens it would just by the nature of musical growth happening for them and all around them definitely be very radical and surprising to everyone.