Not true. A steel boat can last indefinitely if regularly maintained. Properly painted steel does not rust. There are many surviving examples from the 1830s, not restored but maintained continuously.
Properly painted steel wont. But steel wont stay "properly painted" during operation, leading to rust forming between maintenance. Maintenance is also typically rushed leading to painting over rust in many cases resulting in more rust sooner.
We used to joke about the local ferries being mostly rust and paint, but it wasn't much of a joke to be honest.