They won't default. They may play chicken for a few days, but a default would cause economic catastrophe. Trump doesn't care because he is not currently in office and he's an amoral psychopath who thinks he can personally benefit from a catastrophe. At the very least, others won't want to look bad when their voters start losing SS benefits. And it is possible some of them may have actual human emotions where they might get a little guilty upon seeing widespread human suffering. That might be a liitle too much to ask, but I think the former is true.