That's just a theory without any evidence.The planet earth has oil reserves for the next 50 years.
I remember that there was another theory coming from soviet or russian scientists that oil is actually a renewable resource and the oil supply will never end.This is another hypothesis without proof.
So far,the crypto world has no role into eliminating fossil fuels usage.I hope that this will change soon.
Its not just the fossil fules but also take into considreations the material we use for making money like copper, silver and nickel. If the world will use cryptocurrency as mode of exchange for every commodities, the banks would not need anymore of this materials and thus mining of this materials would be minimized. Government could also preserved mountains were this materials could get from to protect the environment. As of now, this is only a theory but i believe that someday it would be in front of my eyes because I want this world to last longer.