Chassis will wear out eventually. I’m not a metallurgist so I don’t know what’s happening to the tubing, but I know the kart will start to feel different and react less and less to adjustments.
You can still be competitive on a local or regional level with a chassis that is a few seasons old but it would be pretty difficult to go to a big race with an old kart. I’ve seen karts that are 1-2 seasons old, maybe 3 at the max, do well at big races with the right driver. It’s tough though.