A massive amount of US dollars is held purely because countries and companies are forced to use it for many types of transactions (eg. trading oil). It's not by choice, they have a gun to head. The US has proven over and over again that they will kill state leaders and invade entire countries in order to preserve that economic power.
When the US loses that power and other countries start using whatever currency they prefer, the USD you are holding in your account will lose a lot of value. It doesn't matter what other countries prefer to use (euro, yen, rubles, CAD, bitcoins, etc.), anything other than the USD will fuck up the US economy really badly. I personally wouldn't trust current US politicians (both sides) to deal with this successfully.
When the US loses that power and other countries start using whatever currency they prefer, the USD you are holding in your account will lose a lot of value. It doesn't matter what other countries prefer to use (euro, yen, rubles, CAD, bitcoins, etc.), anything other than the USD will fuck up the US economy really badly. I personally wouldn't trust current US politicians (both sides) to deal with this successfully.