They only go bankrupt if they don't hold the majority of the mining power, if they do they take over. How is this different from PoS?

Actually in PoS if you try to attack and you don't have a majority you will lose all your coins. in PoW if you try to attack and somehow you miscalculated you will lose a couple of hours worth of electricity after which you can go back to mining normally so much lower stakes for an attack.