Some top ranking sites do not deserve to. When one is lucky enough to be in such a situation it allows us to get away with being lazy, because a site does not have to be too efficient to make money if it is well represented for targeted search queries that send free traffic. But every website has upside potential, even if it already ranks #1.
Improve Internal Navigation & Usability
One client of mine only ranks below his official manufacturer for their name. His site had inadequate internal navigation. I took a day to improve the navigation, and the result was a 150% increase in sales. The last 8 days of last month sold nearly as much as the rest of the month combined. His business model looked like it was about to die, but that one day of work made it functional for at least a couple more years.
High Profit Parallel Markets
I had a site which made a couple hundred dollars a day that was well established in its market, but did not dominate it. Taking the path of lowest resistance, I branched the site into two parallel markets of greater commercial interest where the competition was weaker. On an investment which is less than what the site earns in a month now I was able to increase its income 5x, without even doing much link building.
An Undeserved Ranking
One of my friends is in a high profit market where the competition is absolutely clueless. Basic SEO brought that site to a #1 ranking in Google. The site is highly conversion oriented and makes great income, but now that it already ranks it probably makes sense to reinvest some of the profit into improving content quality and reinforcing that market position. Businesses that do not reinvest eventually fall, especially if they are winning only because the competition is clueless. After spending a couple thousand dollars a day on AdWords eventually the competitors will start to look into SEO.
The Value of Branding
If a site ranks #1, and is monetized via PPC ads, it still might only make a portion of what it should because AdSense is not as efficient as some people would lead you to believe. If a site is strong enough to attract brand advertisers they will pay a premium just for getting their brand seen. Scraper sites and thin content sites don't attract brand advertisers, even if they convert. I have seen a site that was making $80 a month on AdSense make over $10,000 a month selling brand advertisements.