Having purchased many cat trees, and even made a few, I believe the durability depends on the thickness of the sisal rope. So you can either repair the tree with a thicker rope, or purchase a new one made with thicker rope. If you decide to repair it, or make a new one, see the link James suggested What are safe materials for repairing a scratching post?.
While carpet is an option, as keshlam suggested, I suggest sticking with sisal for two reasons: 1) your cat is obviously happy with it, and 2) carpet can have glues and other chemicals that aren't necessarily cat-safe. I wouldn't be worried about carpet that the cat mostly just lies on, but if the carpet is going to be scratched to bits, those bits are more likely to stick to fur and be ingested during grooming. But if you do decide to go with carpet, I suggest turning it around so that the backing is what the cat scratches rather than the pile. It will last longer that way.