Every time I have landed on a airplane reverse thrust is needed, but what if the runway is 18,000ft long. Because if the runway is longer couldn't you save fuel by not using the reverse exhaust
Reverse thrust is never needed.
A reverse thrust is relatively complex thing that can easily fail and for this reason when deciding whether an aircraft can land at some runway, the calculations are done without assuming reverse thrust. And of course the stopping distance for aborted take-off is also without thrust reverser, because if your engine failed, it won't provide any.
The reason to use reverse thrust is to reduce wear of the brakes and tires. The kinetic energy of the aircraft is huge and when it gets converted to heat in the brakes, they can get pretty hot. So if available, pilots always use reverse thrust to ease the brakes.
But it's not a big deal if not available. Thrust reversers are on MEL (minimum equipment list) for most if not all aircraft meaning the aircraft can be dispatched with thrust reversers inoperative. The brake wear will be higher from the more intensive braking, but it is better than having a plane wait for spare parts at some remote location.
Regarding long runways, there is almost always a possibility to turn off the runway earlier, which often saves time for the aircraft (shorter route to taxi) and makes the runway available for next aircraft sooner. So pilots usually still want to use some reasonable braking.
Aircraft don't need thrust reversers on a long runway, in fact they often don't need it at all. It just helps in decelerating, reducing the wear and tear of the brakes and tires. That comes at the cost of fuel, but it saves on downtime replacing brake discs and tires.
Another reason to use the reverser is that sometimes it saves taxi time to take an earlier runway exit. This saves fuel and time = money.