Slapping stickers on something is not a scaling issue. The secure storage and transmission of private keys is. The idea that you'd just slap the private key on a sticker and have no other record of it is not something I even considered because that's very user-hostile behavior (unless they already have the intention to mod the device software before they buy it, they're unlikely to retain the sticker). It also screws with the secondhand market, both because secondhand devices won't have their key, and because the original owner obviously had access to the key and could have tampered with the software on the device before selling it.