There's going to be a minimum cost though and it's higher than 20$ per barrel. At best, producing oil at those price levels is going to be not very lucrative. A lot of the remaining oil out there is getting quite hard to produce meaning the cost is high. E.g. Shale oil becomes a loss leading venture when the prices drop that low. This wikipedia article suggests that 25$ is sort of the lower boundary here: https://en.wikipedia.org/wiki/Oil_shale_economics. I have no idea if those numbers are correct but they are probably much higher than 20$ per barrel.

There is of course plenty of oil left but it is not the same cheap oil that we were used to last century where you basically stuck a pipe in a ground in a place like Texas and cheap oil would come out. Most of that oil is gone. Also, not all oil is created equally and some oil is a lot nicer to work with than other oil when it comes to refining it. Which is why the US still imports oil from the middle east. Refining is not free either obviously and actually uses lots of energy. Ironically, a lot of wind/solar is being used for that.

This is indeed not news. Oil investors have known this for some time and have been voting with their feet. Also, most EV owners are well familiar with the fact that the cost of recharging is peanuts compared to filling up a car with petrol/diesel. I'd say that 20$ is going to come down by 2-4x over the next 20 years.