How's this for a theory?
The Saudis perceive the beginning of a gradual decline in oil demand initially due to the economic turn-down (China) which will be continued to the move towards renewable energy. At the same time we've seen lots of new and expensive oil sources explored. Without action this would lead to a long-term over-supply of oil, which would leave the Saudi's with unexploited reserves. Due to their cheap oil and huge financial reserves they can weather a storm of cheap oil longer than all their competitors. So they deliberately hold the oil price low in the medium-term until almost all their competitors shutdown. Once shut-down infrastructure, people, skills are lost and hard to restart. This leaves the Saudi's the main global supplier of oil into its twilight years, allowing them to control price and monetise all their reserves. If so, I wouldn't be betting on a return to $100/bbl prices anytime soon.