Which country would benefit from a weaker dollar? Germany wants to sell their cars. China their phones. If Americans can't afford them any more, then a big market would disappear.
US consumers will get fucked with a weaker dollar. Imagine having to pay $7 a gallon for oil instead of $3.
There is another consequence. A lot of other countries will become "rich" all of a sudden. Which means they won't use US dollar as a medium of exchange and America would have to "earn" by exporting. Which means people will have to work a lot more to produce more for exports without consuming ourselves.