>Someone tell me why this couldn't happen.
Beware the ides of march.
During a constitutional crisis that doesn't seem to have an immediate resolution political violence should be expected from the inside as an attempt to reach a resolution. The last line of defense is that military leadership actually do have a pretty solid loyalty to the constitution and soldiers are pretty well trained to follow the chain of command.
nobody can gain loyalty anywhere near trump nor does anyone have close to the unhinged charisma
but the government is shut down there should be no expectation that there will be any agreement to half fund it and absent that there's not really any foreseeable mechanism for the treasury to start operating on a large scale entirely outside of the law
so i guess the other last line of defense is the bond market and foreign exchange markets which wouldn't respond well to dictatorial control of the treasury and the fed