If you don't want apps from outside the store on your phone then don't install any.
External apps should obviously require the device owner to authenticate in order to install. If this can happen without the device owner authenticating, that is a security problem, but so are a thousand other things, like copying your data off the device, that are no problem at all unless they can happen without authenticating.
External apps should obviously require the device owner to authenticate in order to install. If this can happen without the device owner authenticating, that is a security problem, but so are a thousand other things, like copying your data off the device, that are no problem at all unless they can happen without authenticating.