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?
This is far from reality and there's no such dump over night, I don't read it on the price history of the whole market. There are times that we are suffering from the big inflation and as far as I remember, bitcoin created right after the financial crisis in 2018, and its purpose is to become the best option if crisis happen again. Now, most of the country enters into a recession yet bitcoin is still not that high, so the value really depends on the demand in the market and the value of bitcoin is unpredictable.
Well, US Dollar has been constantly losing value since his creation. US Dollar actually designed to yearly lose 2% of it's value, and now FED is struggling because they have been failing at this during the last years.
So don't look for overnight dump (yet) but the continuous bleeding in your purchasing power.
Also, it might be a typo, but Bitcoin was created after 2008 financial crisis, of course.