Is knee rehab enough without knee surgery for bad knee cartilage?

Depends. It depends on whether you are referring to articular or meniscal cartilage (there are two types within the knee). Many factors influence a recommendation on this question. Your age and degree of arthritis would need to be considered.
It depends. "bad knee cartilage" can mean a small cartilage tear treatable with arthroscopy or bone-on-bone arthritis that calls for a knee replacement. Regardless of what your specific problem is, doing physical therapy, engaging in good nutrition, and maintaining your ideal body weight will help. If after rehab you continue to have pain interfering with what you want to do, you might need surgery.