In a small company the buck always ends with the owner. If the company fails if someone leaves then that is either a failure of the owners ability to manage different aspect of the work or poorly run company.

An employee can at any time get a medical condition that puts them out of action with month long recovery time. A car crash, a bike accident, or any other problem that results in surgery. Maybe they get serious sick. Its quite rare that people are perfect healthy and remains so for decades, so it doesn't make any sense to risk operating a company on that condition.