Ops point was that if the cost of fuel drops by 75% (using the numbers from their comment), then things like sails may no longer be cost effective. For example

Ops point was that if the cost of fuel drops by 75% (using the numbers from their comment), then things like sails may no longer be cost effective. For example, it saves 20% of your fuel cost, great! But its cost is 10% of your fuel cost. At current fuel prices, you still come out ahead, since you are saving more than you are paying. However, if fuel is 1/4 the cost of now, then you still save 20% of fuel cost, but the sails now cost 40% of your fuel cost… aka the sails now cost you money to run. To be clear, while these sails are profitable, they will be used. Op is just saying that they think they will become unprofitable sooner than expected.