if the site's content remains the same, they tend to stop looking at it
One of the most-overlooked parts of a website is that it needs to be maintained. Customers visit sites multiple times and if the site's content remains the same, they tend to stop looking at it. After all, it doesn't seem like it's going to change. Another reason for changing content is that it often lends credibility that your business is growing and maturing. Customers don't want to do business with a company that seems stagnant and unchanging.
this is balanced against how much you may lose if you update nothing
The cost of maintaining your site depends on how your site is setup, if it has a database backend and how often content is added or changed. Things such as changing text or images, adding/changing products or pages, up to completely changing the entire look of the site, costs time and money. The more often you update can cost more money, however, this is balanced against how much you may lose if you update nothing.
website maintanance means keeping your site in-line with your business
Unless you're running a blog or news site, keeping your site fresh doesn't necessarily mean hiring a full team of content developers to constantly put new content on your site. Normally, website maintenance means keeping your site in-line with your business. If you add a new product or service, or change a price or add a discount, you would want that information on your site.