It just seems weird that the total valuation of the currency (20,738,252 USD) is worth ~10x more than the price it might cost to potentially ruin it...? 
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.