I wouldn't yet cut or throw out the old quilt, but I wouldn't "repair" it either. Over washing or the products you use, dry bleach can take years off a quilt. I tell people that while my quilts are designed to be machine made and machine washable tbut hat cotton is a natural fiber and it is only natural that it was change with age and use
I have put on new tops or backs over the familiar favorite quilts, incorporating a tattered worn out blanky inside a quilt going to college.. My husband just destroyed a quilt I made him before he moved to be with me... long story but he got threw it into a bag with some stinky stuff and the smell transferred over. He freaked out that I would be so mad at him and instead of telling me and me doing some scent control, he washed it like 7 times in a row and just thrashed the vintage fabric I used for the back. Here it is 20 years later and with the new back the quilt is still with us and made the move.