Like many powerful currencies in the past, USD may fall to ground too, my question is will that effect BTC value? Or is it independent of all other real world or non digital currencies of the world?
Bitcoin depends on supply and demand solely and nothing else.
USD has nothing to do with bitcoin, as neither do all other fiat currencies, prices for electricity, the cost of oil/gas, etc.
For example, mining in Japan is several times more expensive than mining in, say, Switzerland - so that mean that "Japanese bitcoins" should be more expensive that "Swedish bitcoins"?
No! So that's the same correlation (that is, no correlation at all) of cryptocurrencies with the objects of the real world.