Cash will probably remain for the next two decades at least, but by then, things might have changed a lot to a point where we can't visualize right now.
You right mate, and don't think that Fiat will become extinct in the future. Yes, we accept the things that changes may happen in the coming days but I don't feel of losing fiat value in the market. Bitcoins popularity is not the reason of losing but it depends in the numbers of people are currently using it.