Sails (or “rotor sails”) are not likely to be the only mode of propulsion on large ships in the future.
They will help reduced the fuel consumption from conventional engines.
The requirement to reduce green house gas emission is the driving force behind the development of alternative ways to propel ships, cars, truck, buses etc., not cost of crude oil.
At least that is the case in most of the world and for responsible multi-national companies, like Maersk etc. Even some oil companies are getting on the band wagon in deed, not only in talks.
Whether they are doing so from guilt, newfound conscience, public pressure, or because it is good business can be debated. But it doesn’t matter as long as they do the right thing.
One thing for sure, countries and companies that lag behind will loose out in the long run.