I've got a set of Conti GP4000 with some cracking on the sidewalls. I can't see the tubes underneath, but some of them are more than hairline. There is no cracking or splitting on the tread, which is really still in great shape.

I ask the question more for the curiosity of it. I'm going to change the tires anyway. I'm just wondering, as a general matter, if that cracking means the end of the line for tires so I know if it happens again.