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?
It depends on how much of what sort of things are being offered and on what terms. You probably shouldn't give things to strangers that are a substantial fraction of the cost of controlling the network. If someone was holding a lot of coins and did a big attack it would probably shake faith in the network and reduce the value of his coins. And he probably can't easily spend them all more than once and pull them back since word will get around fast and the most vulnerable will shutdown or reduce payouts (like ATMs and similar instant stuff).