Firstly, this has nothing to do with $1 tickets to get unlimited connecting pax.
Consider the following scenario:
Have a hub in airport A.
Open a route A to B, oversupply it by a lot, making most of the pax blue.
Open a route A to C, oversupply it as well.
Now the question, would route A->B now has less blue pax because they are used in route A->C? Or are they completely separate and draw from different pools of connecting pax?