Assuming they can maintain that level of supply as demand drops. Some the economies of scale they operate with will be lost as demand slows and then the logistics that requires large and ongoing investment may slow considerably, although much of the latter is probably outside the 20 year horizon.
It could play out very differently on the consumer side though. If electric vehicles became 50% (or some arbitrary significant percentage) of the market then a lot of local gas stations will close, the ones that remain may have to start charging higher prices (again losing the logistics and scale advantages of today). There may be some tipping point where the whole retail side of the industry finds itself in a death spiral where gas stations are charging more and in less locations, making oil powered cars less convenient and driving more people to electric vehicles.
Yes they will. I'd bet that the price of oil is not going to go above $80 or so in the next twenty years.
Two major reasons, and a bunch of minor ones.
1: massive quantities of non-OPEC oil are available in shale and tar sands. Right now these are only barely profitable, but if the price of oil increases production there will increase, stabilizing prices.
2: renewable energy is now just barely cheaper than other forms of energy, but they're projected to drop dramatically over the next years, radically transforming the energy market.
Some uses of hydrocarbon cannot be easily replaced by renewable electricity but most can. When a cheaper substitute is available, demand and price drop.
The only way the consumer price of oil is going to go up over the next few years is if governments come their senses and put in place a carbon tax or equivalent. (And that's reason #3 -- the environmental movement will demand that the world use fewer hydrocarbons, forcing less demand, forcing prices before the carbon tax to go down.)