Your developing process with opensource model will be much faster, you will always be few steps ahead with your community.
Theoretically yes, but those people who will fork and "steal" your project are not your clients you are not targeting such audience, they will never pay for your product (hosted service, support etc...).
If they are capable to fork you project, improve and change you code, develop the product faster then you, become your competition, you should hire them :). This is a signal you are failing, something is wrong in your organization/team and you should quit.
Ask your self,Do i ever heard for such story ?