You can already hand a binary that runs on the CPU and let it do whatever it wants, the problem is that said binary needs to interface with other components of

You can already hand a binary that runs on the CPU and let it do whatever it wants, the problem is that said binary needs to interface with other components of the system. Specifically it needs to be able to ask for input, modify said input and output it somehow. It is those points, where it interacts with other sub-systems to display things to the user or receive input from the user that things get dangerous. Even with IPC there is the possibility that there may be a security issue processing the content. NaCL and others try to solve part of the problem by making it easier to sandbox the application by limiting what it can do, but there is still an interface into the real world.