The search engine optimizer’s work is almost never done. Only in very rare instances have I seen sites that have been optimized for every possible keyword phrase and is ranking well for all of them. But I also know that websites can reach a point where the vast majority of the optimization work has been done. All the low-hanging fruit has been gathered and the primary keywords have been optimized into the site properly. All that remains is the “upkeep” of the optimization efforts, the tweaking to squeeze out better performance and blazing new ground into the long-tail keywords.
Many site’s reach this point of the optimization campaign while many of their keywords are not quite ranking as they hoped or want. This is the point of diminishing returns in SEO. More work won’t necessarily improve the rankings in proportion to the effort it takes.