While living in Spain, I worked on many small businesses, mainly Estate Agents who wanted their websites quickly optimised and as cheaply as possible. The only thing that was important was to appear at the top of Search Engine Results Pages (SERPs) for the keywords they wanted. Once they were number 1 they thought they had done all the hard work and now there was no need for anymore optimisation.
SEO is ongoing
You can be sure that any website that reaches the top ranking on SERPs for their keywords will be working damned hard to stay there. No matter what service you offer or what product you sell, you will have competition online for the keywords you want to be found for.
This competition will always be trying to beat you to that top spot while new competition will be appearing all the time. Not only that, but search engines are continually changing their ranking algorithms. Not all of these changes will have an effect on your top ranking, but you can be sure over time, if you are no longer improving your optimisation and link building profile, then you will lose that number 1 ranking to a business that is working harder than you to get to that top spot.