I am not totally sure about national currencies but threat in job is absolutely possible to happen. Actually it is happening right now in the current scenario. I've known a lot of people who left their regular job coz they are already earning so much money here compare to how much they earned on their job.
If people leave their permanent job in order to deal with the crypto currency, this is vice versa, means that the crypto currency creates jobs and therefore does not carry any threat to the existing jobs.
There is no threat of bitcoin and other decentralized crypto currency and national money. Decentralized crypto currency can only squeeze the national money a little, but does not pose any real threat to them, because the crypto currency is able to walk only in parallel with the national money and in no case can substitute them.