when the time comes that USD would crash,it would affect all the economic state of each country since this currency is one of the major pairs in Forex. It would also affect the price of btc since USD dropped it's value.
Yes to us dollar is major currency. But apart from that there are pound, euro, Kuwaiti dinar,etc. So it will affect the world economy but it won't have much affect like decreasing other currencies value by too much plus value of btc will increase.