Hard to predict. Either way this may take a lot longer so I don't think we have to worry about it now but it's nice thinking about it. By the time the last one is mined I believe we will have many other coins that potentially could replace bitcoin or we may not. With the last one mined I believe the prices will keep going up.
I believe that the price will increase even more due to the shortage in the market and the demand will increase even more so bitcoins do really have a bright future.