If the traffic is encrypted how do you guarantee that the device doesn't spy on user and doesn't have backdoors preinstalled? And why would a device need full

If the traffic is encrypted how do you guarantee that the device doesn't spy on user and doesn't have backdoors preinstalled? And why would a device need full Internet access? Shouldn't it be only accessible in home network with maybe some (user selected) servers from Internet it is allowed to connect to? We could use some sort of VPN (from device to other owner's devices) so that even if it is connected to Internet directly it doesn't accept any packets coming not from its owner. VPN also provides encryption while allowing user to examine traffic in clear text. I think we need some type of easyly deployed, ubiquitous, simple, snandartised VPN type protocol so that the users can build protected networks over public insecure infrastructure. Today if you connect your device to Internet you connect it directly to NSA and chinese hackers. And if your device is disconnected from public network hackers cannot break into it even if they have exploits.