A 51% attack doesn't allow you to steal other people's coins, nor does it let you easily alter deep historical transactions. To rewrite the past, an attacker would have to continuously outpace the rest of the network to rebuild the chain from that exact point forward. The primary threat of a 51% attack is that the attacker could double spend their own coins or censor or block specific transactions.

I've actually wondered how many datacenters it would take to effectively perform a 51% attack on bitcoin.

It seems like most newly built computing resources are at the disposal of a few companies and a few people...