Thats just like Bitcoin is USD, and all the altcoins are currencies of other countries. Now if an altcoin crashes, it doesn't affect the countries currencies. But if value of bitcoin will always be in dollars and then to the currency. So in a way if dollar crashes, then you will see a crash there too.
Not clear what your saying. The dollar is like any other currency, if it crashes then the price of Bitcoin measured in Dollars will shoot up. Yes many of us measure the value of Bitcoin against the dollar but it doesn't mean Bitcoin will move with the dollar. The ideal way to measure Bitcoin long term, as in over decades and years is probably against Gold and Silver actually.