Each blockreward is 25BTC=$11,000
It would cost at least $55,000 for someone to unspend a txs that is buried 5 blocks deep.

What if Satoshi decided to move $1,000,0000,000 of coins at once?
It would take 90,909 blocks of confirmations before we could trust the blockchain again, 631 whole days.

Before then, it would still be profitable for satoshi to buy massive amounts of computers, and to build a fake chain from before he spent the money.

If satoshi did build this fake chain, it would undo every transaction.

If someone spends a massive amount of money, it leaves bitcoin in an unsecure state for a long time.