Corporations are answerable to their customers, their shareholders, and the law.
Your premise that they are only answerable to their shareholders is simply false.
But regardless - you have articulated no catastrophe for society.
All you have stated is an ideology about power itself being bad.
If you break up all the big US companies, there are plenty of big companies outside the US who will happily take their business and power.
Corporations are answerable to their customers, their shareholders, and the law.
Your premise that they are only answerable to their shareholders is simply false.
But regardless - you have articulated no catastrophe for society.
All you have stated is an ideology about power itself being bad.
If you break up all the big US companies, there are plenty of big companies outside the US who will happily take their business and power.