What's supposed to stop open source OSes from including a copy of your commercial function in their next version? Isn't recreating commercial products for free

What's supposed to stop open source OSes from including a copy of your commercial function in their next version? Isn't recreating commercial products for free of the point of open source? By your definition anyone who doesn't own the OS is a sharecropper. The real danger is in writing software that is easily recreated. In the end, all software is made obsolete by better (or better marketed/distributed) software. The only difference with the OS is that it's a little faster.