We see the danger of returning soldiers marauding in their own country in Germany after World War I and in Japan by the Ronin. We know the danger to the Roman Empire from their hired mercenaries at the gates.
The danger is not in what Trump is doing now in Saudi Arabia, the danger might arise when hiring American youth out as mercenaries becomes a decision made by unpatriotic Democrats. What assurance will we have that they will not put a Joe Biden in charge and close an eye to his raking off blood money?
Worse, will Trump's successors cut deals to wage wars for profit? Which Democrat do you trust?
I imagine he sees it as similar to expecting South Korea to pay us for the troops we have there.
The question isn’t so much being reimbursed for it as how much we should be projecting our military strength with troop placement around the world.