You don't need 51% of the world's power though, just 51% of the power of people who care about how the system works. Most people using block chain cryptos don't care at all, so the threshold is a tiny percentage of the user base.

This would just create a fork in the blockchain where 51% of the network doesn't match the correct state of the blockchain that the 49% have. The 49% would effectively stop working because they could never validate the transactions that the 51% takeover has falsely created. The node operators of the 49% of the network would need to reach consensus for how to deal with the problem, but essentially they would just adopt code that ignores the 51% data, so they could continue to process blocks of transactions. Without manual intervention the 49% would be frozen. The 51% is just fake, they haven't really changed anything because every real node operator would know it's false data.