Because they'll go somewhere else, and probably use some other currency, which will weaken the dollar in the long run and god knows what will happens once the US loses its de facto world bank status. This is not the straw that will break the camel's back yet, we're probably far from that, but it's one more event that goes in this direction.
The point of controlling the world's financial plumbing would seem to be that you can use it as a weapon, but paradoxically if you ever actually do use it as a weapon, it stops being one. Seems like maybe the sword was made of butter the whole time?
