If crude oil is run out, the whole civilization is affected very much. Nowadays, many products are come from crude oil. Polymers are from crude oil. The paint is also from crude oil! Lubricant is also the same problem. At the same time, the matels are also nearly ran out. So using liquified hydrogen, fuel cells, vegetable oils, electricity, solar cells cannot solve the problem. The problem is what materials should we use to build aircraft? Let us using hot ballon again! Takes several month to travel. It seems interesting.
Finally, I hope the scientist is under estimate the amount of natural resources under the ground