I replaced the cush drive in mine a few years ago. It wasn't difficult to remove or to replace.
Yes, it was worn compared to the new one I put in. Also had stress cracks. If you pay attention to your rear wheel, you might see black dust collecting in spots. That black dust is coming from the cush drive. I would think the more you thrash a bike, the more likely it could fail over time. If you are a more gentle rider then it would probably last the life of the bike. I just did mine because I was concerned over the dust collecting and I was going to have the wheel off for a tire change.