"We never needed to spend money on website maintenance before, why now?"
Today, the web is different than it was a few years ago. Instead of websites being written simply in HTML without any easy way to update them, websites today mostly use databases to allow easy addition of fresh content. These databases involve several programs that are often updated mostly to improve security.
Content management systems like Joomla and Wordpress are updated several times a year and each time all the extentions used to make these programs more usable require to be updated as well.
On top of that, websites today compete for top spots in search rankings with specific SEO efforts required. To maintain these ranking, SEO efforts need to be continued or the ranking will slip and eventually disappear. With no new work done on SEO and no new content added, search engines like Google will consider you website 'old news" and will rank other websites with newer content higher than yours. Eventually, your website will disappear off the radar.