The human soul is insatiable. I do not think anyone can get to that point where they think they've achieved everything. This is because within every human are limitless potentials. This is our driving force--knowing that there is something out there we are yet to explore; a void that needs to be filled. If we lose this drive, we lose our essence of living.
That is why it is important that we keep reinventing ourselves by learning new things. There is so much out there for one to feel that they have done it all. I will take Arnold Schwarzenegger for example. From being the world's strongest man to becoming a actor and then a governor. He found new ways of expressing himself and I think that is how we should all live. We all have limits, however, we can still explore as much as we can. It's not about being a jack of all trade and master of none. There is time for everything even your career path. When that time is up you need to find a new path. Lingering too long in a place because it's familiar has been one of the limiting factors impeding the growth of many people. And even in your said field, you can discover new ways of using your knowledge. So you might not have to drop it totally, but further the knowledge of that field.
If I reach certain success, I will still work hard to find new ways to develop strategies for the business to advance to the next level. I guess, I would be more likely to focus on one business. I don't think I would drop any business if it is sucessful as I would treat it like my baby and seeing it grow up is something that Is fun.
If a person constantly look for new challenges just as the business starts to be sucessful, there comes a saying. A stone that rolls down a hill collects no moss. Why would anyone want to just let go of a business that is sucessful and go for a new one from scratch.
The only reason I feel is right for letting go of a business is when there is an industrial change. If the industry changes and at some point of time, the business is no longer relevant then, it is time to change and shift to a new business..