Success doesn't mean you can take it easy...success just means you have higher-grade problems.
I attended a board meeting today for a startup. The team had made impressive progress in nearly every area. We rightly celebrated their achievement. And then we told them that we wanted them to step up their game. "Hey," was the reasoning, "Now that you've made it this far, let's really work on some stretch goals."
I recently lost 30 pounds as part of a weight loss competition (Ed: Which unbelievably, I did not win). That just meant that I decided I needed to lose another 15 pounds to get down to my college weight.
Ask Warren Buffett if his incredible success makes his life problem free--of course, being called upon to save the financial system is a high-grade problem to have.
And while I'm very much against the hedonic treadmill, I'm glad that success doesn't decrease motivation.