lets say usa dollar and all currencies will be highly devalued .....then how we know the btc value??
most traders use USDT so if something happens with usa dollar is my USDT Will be worthless over night?
No, usd dollar can never be worthless. It always has its own price either high or low. Many countries are depended on this currency and as you observe, it is the most value currency in the world. Moreover, the monetary system has been developed for century. Precisely banks and the government understands how to maintain the power of cash in the 21st
After all, people cant live totally without cash. Thats why dollar can never ever sudenly disappear