They don't need to pay for removal of competing content directly, because people only have so many hours in the day. If they pay to promote favorable content, the unfavorable content will be crowded out.
But "people" isn't just a single thing, it's a distributed system of many interdependent but ultimately independent agents. If you only grind your own axe, yes many people will be indeed swept in, but a lot of other people will spend their parallel hours seeing others grind their own axe, which would include the axes of your opponents. There is an equilibrium where you reach all the people you would ever reach but your opponents still carves out their own niche out of other people's attention.
Whereas, if you pursue both pushing your agenda and suppressing your opponent's agenda, you will more truly make it zero-sum. Now even the parallel people who would never see/care about your own propaganda would also never see your opponent's propaganda, because you suppressed it at its source.
Whereas, if you pursue both pushing your agenda and suppressing your opponent's agenda, you will more truly make it zero-sum. Now even the parallel people who would never see/care about your own propaganda would also never see your opponent's propaganda, because you suppressed it at its source.