There is no pax demand created by operating a service and no ability to create demand by, say, offering seats for $10.
Demand is what it is. It does change on a global scale on a monthly basis per real-life statistics, but those statistics are averages for the globe, not a particular route. So a given route may go up a couple percent a year most years. A noticeable effect over the course of an entire game, but barely a blip on all but the largest of routes over a few-year timeframe.