That's not what happens. 51% attack can undo the last blocks and replace them with new blocks, but the blocks still have to be valid. They can't edit the blockchain and transfer someone's bitcoins.
That's not totally true. If you look at what Bitcoin Gold did (I believe that's the fork that did it) they actually reorganized the forked chain to unlock satoshi's coins for their dev team. This was also Craig Wright's plan for satoshi's coins on the BSV chain. He believed if he could convince the world he was satoshi, he could order miners to go along with a new fork where satoshi's coins had been given to him. Sure, this is insane and nobody would support a reorg or a forked Bitcoin chain to move coins around, but it is definitely possible. Maybe someday in a distant future people will decide to fork BTC to unlock satoshi's coins for some good cause. I could see this happening before increasing the amount from 21 million to further block rewards.