You don't get everything. In fact you get almost nothing. You can rewrite recent blocks which just means that you can unpay coins you already had control of. Or you could refuse to put transactions in blocks or to build off of anyone who does effectively a DOS. But you don't get $20M by any means.
How about if someone with a lot of bitcoins rented a machine like that out for a short period?
You could then cash in on the bitcoins several times over?
What would/could happen to bitcoins after such an attack?