What do airlines do if a flight is almost empty, would they cancel it?

airlines launch reduced fares

THEY WILL ADJUST THE PASSENGERS IN OTHER AIRLINE OR OTHER FLIGHT EARLER OR DELAYED . BUT FLIGHT WLL NOT BE CANCELLED. 

In my view, cancelling a flight will send a wrong message to their passengers.

Now a days these airlines have an monopoly of selling ticket till last moments, which is not good. I have seen many flight's going with spoilage. Instead of getting more customers they care about more fair which is not good for their future business. They should give an offer of fair reduction before atleast 24 hours. So that they can fill seats with minum profits but they will get more new customers and will not loss as spoilage.

It is not possible,But can be canceled if technical malfunction occurs

In the airline industry, cancelling a flight due to commercial reasons is literally considered as a last option. If we look at how airlines manage this challenge and consider the costs of both spoilage and denied boarding, we’ll successfully arrive at a meaningful conclusion.

Spoilage is the term used to refer to seats on an aircraft that are flown empty as a result of either no-shows or pre-departure cancellations. Airlines typically measure spoilage after the fact by looking for flights that sold out but departed with empty seats. They attempt to avoid spoilage for future flights using the process called overbooking.

Based on historical data, revenue management department is responsible for analyzing and forecasting the no-show and cancellation rates for each flight then sell seats at a level expected to cause the aircraft to depart full. Obviously, by doing this the airline runs the risk of having more people show up for a flight than the aircraft can accommodate. If this occurs, the airline is forced to deny boarding to customers holding confirmed seats.