I guess you could darn them. I would probably first fuse on some interfacing on the wrong side. Then use your sewing machine to sew over the disintegrating area. Sew back and forth, around and around, a lot of times, using colorful thread. it could be random/distressed looking, or embroidered-shape looking. Avoid too much of a defined perimeter because that requires more structure than the delicate jeans fabric is allowing you.

I think maybe the jeans fall apart this way because they are stretch fabric and the elastic fibers fail faster than good old cotton. Wash your stretch jeans in cold water and cool dryer/line dry.

Cheers!