Copyright notices along with page modification dates are things that Google looks at regularly. If there are no updates, and someone else in the same vertical has updates, why should Google show your site.
From an SEO perspective, it's critical. From an overall business side of things, it's not all that needed. We worked with a roofing company that has been in business for 45 years. Their copyright still says 2010. Not a change on their site since that time, and in their eyes, they don't need any changes. The site is treated as a brochure just to lend credibility to the business, but they're not "looking" for new clients. They already have all the clients they need/want, it's purely a business card so people can find their phone number.
