51% lets you censor transactions. You need more than that to reverse a week old tx. Way more. You have to outrun the rest of the world and gain a week of advantage. So if you have 90% of the hashrate, you go 9 times faster than the honest miners. Every day, you can erase 9 days of blockchain. But that 90% means that you build a mining farm that is 9 times greater that the current hashrate. Redo the math on the costs and logistics of that.