One approach that might be reasonable is for device makers to provide a hypervisor to control actual hardware access, and then implement their environments in v

One approach that might be reasonable is for device makers to provide a hypervisor to control actual hardware access, and then implement their environments in virtual machines running under that hypervisor. iOS then becomes just one environment that the phone's owner can choose to run under that hypervisor. Within any given environment the maker of that environment should be free make it as open or closed as they want. The device maker should have to make it easy to run different environments.