hi, i dont think BTC will replace dollar, at least in 5 years time. Because people involved in cryptocurrency are still not many as compare to the world population.
hi, i dont thin the Fiat Currencies will crash, as till today not many people are into cryptocurrencies, and the fiat currency like US$ is still dominant the world. So, dont worry, the fiat currencies wont crash!
I think it's a good idea for bitcoin to become a common use currency and starting from those online shopping sites, like Amazon, eBay etc. But not just for BTC, also for ETH as well.