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Unless the protocol itself is changed where it's possible to move coin without private key.
How can non-mining nodes change the Protocol?
UASF, UAHF, use client which follow different protocol, etc.
Non-mining full-nodes cannot change the protocol. This can only be done by miners.
You want to say that full-nodes can put forward a condition that blocks will be correct if it is possible to move coin without private key. And it will force the miners to change protocol ( shoot yourself in the foot). Fantastic scenario.
