We can adapt and the process of doing that in capitalism involves prices rising and rising until people can't afford things and stop buying them, thus reducing demand.

That's super tough on people who don't have much money and they don't consume much anyway so when they tap out it doesn't reduce demand much. So prices need to rise enough to hurt the middle class in developed countries, meaning the lower classes everywhere else have a really shit time.

Meanwhile there are some oil uses that are completely unable to be reduced, such as emergency services, food distribution, etc so the govt will intervene in the market to ensure that happens. This means all non-essential sectors of the economy must reduce usage by significantly more than 20%.

Meanwhile every country's govt is doing everything it can to try to lock in 100% of their usual supply and some will succeed, leaving other countries to make much bigger cuts than 20%.

There will be lots of people making 100% cuts while a few make none. Humanity isn't great at sharing especially at a global level.