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.
As the dollar collapse, people will flock to Bitcoin as a modern safe haven asset. Consequently, Bitcoin will see an eruption in purchasing power and global acceptance, quickly decreasing the value of other currencies and be at the top of currencies.
The dollar is quite strong. With the euro and the pound in some trouble the dollar is a safe have for forex traders.
If it comes under attack bitcoin may profit but there is no guarantee. Brexit did not boost bitcoins price for example.