They can't edit the blockchain and transfer someone's bitcoins.
Reversing transactions
is transferring someone's bitcoins. Stealing miners' rewards
is transferring someone's bitcoins.
Maybe someday in a distant future people will decide to fork BTC to unlock satoshi's coins for some good cause.
Never going to happen. That'd violate censorship resistance. Also: Satoshi's coins are most likely going to be moved within the next 2 decades, probably, due to them being paid in public keys.
The reason why 51% attack doesn't work is because how decentralized the mining in bitcoin is. It is not possible to gain control of that much hashrate.
And even if you did, it'd be much more profitable to just earn fees and block subsidies. This is where the game theory depends anyways.