All Trump has to do is quietly let it be known that anyone who supports Preibus is politically dead to Trump.
Preibus will resign because those who elect him need the POTUS to help their respective states.
Trump knows that he does not have to “fire” someone to make them go away for 4 years. He just has to let it be known that they are radioactive.
Most CEO’s know that they rarely have to fire someone — just let that person know, directly or indirectly, that none of their suggestions or programs will be funded. Only those with golden parachutes stick around to be fired.
You think anyone on the RNC cares about that? These are not elected officials - these are party insiders. Trump has no control over whether they are on the RNC or not, so he has no leverage over them.
And as has been pointed out before, Priebus was not planning on running again, so he will be leaving after the election is over. But the chances of a Trump supporter getting the job are slim and none...