Bitcoin price isn't pegged to just dollar and I don't know what might happen to dollar accepted countries if Bitcoin drops to zero but I think it might increase the demand for Bitcoin which possibly might increase the price of Bitcoin as well..
Remember Bitcoin is a decentralized digital currency and as such has several other importance like serving as means of payment, investment and also as an asset for the future.
Just as others have stated that one of the major reason why most currencies loss it's value is the excessive printing of those currencies making it's supply much more than the demand and you should also know that Bitcoin has a limited supply and it's volatile which means it can easily fall or rise.
Yeah it goes that way, right now fiat is getting less and less valuable and it has been for the past 3 years or so, and we just need to realize that bitcoin gained value thanks to that. Back in the day 20k was our highest price in 2017 December, and up until 2020 we haven't seen above that, and right now we are "crashed" at 28k, that alone should be a big example, we reached as high as 68k which is a proof that we have done as well as we could hope for.
That's more than enough for me and I think that dollar (and all other fiats) will lose value and we are going to end up with something much better for bitcoin price, the more these fiats lose value the more price bitcoin gains and we will break above the current ATH very soon, in a few years time.