This does not work if your communication enpoint is the same as your encryption endpoint.
Or you don't control your key material.
Or your tech supply chain.
Or leave your device unattended.
Or aren't susceptible to the same "five dollar wrench" attacks used by certain in-person Bitcoin wallet thievestgat are also available to state actors.
I could go on...
You may never be able to completely protect yourself, but you can make an attack much more costly. For example, I use Heads with a hardware key to defend myself from an Evil Maid.