If Russia is on the verge of going bankrupt...
The Chicoms will buy more oil for their reserves (its probably a safer investment that loaning $$ to Uncle Sam anyway).
And as Pat mentions, nations around the globe, from Venzuela to Iran depend on oil sales....not to mention all the Arab nations that have absolutely no other export other than oil, and happen to be in the midst of popular uprisings/civil war/ISIS threat. They can’t hold out for long with the lower prices. I don’t think the Saudis are going to be able to pull it off this time.
They have sovereign wealth funds worth at least 1 year's GDP. They can ride out $50 oil by producing more barrels of something that costs them $10-$25 per barrel to extract. Oil service companies involved in fracking and deep water projects that were going full throttle with large debt issues and dividend payouts will go bust.
Now, you might say that new oil service companies will sprout like mushrooms when oil prices start rising again, so the Arabs will have wasted their ammo. The reality, however, is that the rebirth of these companies - via the veterans of the ones that went bust - will take time. Financiers will be leery of jumping in until they're confident the Arabs are done punishing them. In that interval, Arab producers will recover much of what they lost during the price war.