This is not entirely true.
Gresham's Law states that bad money drives out good only as long as there is a law requiring people to use the "bad" money.
Since the Bitcoin is decentralized and independent, and therefore by its very nature "lawless", it should drive out all the bad (fiat) currencies. So states Gresham's Law.
I think you reversed things mid-stream....yes, BTC is not being forced on anyone....but USD / EUR / etc *are* being forced on people (via Legal Tender laws). Therefore this is why good money (like bitcoin / gold / silver) is being driven out of the market.
People will naturally choose to first spend all their 'bad money' in such situations and hoard the 'good' money. This is exactly what we see with regards to BTC.