Cash will never go away. I am sure that is true. Reason:
1 / Not every country is modern enough not to use cash.
2 / The world is still very poor and they only use cash.
3 / Cryptocurrency is facing the challenge of being hacked and used to serve money laundering purposes.
And it will take a long time before everyone in the world is able to have some kind of computer .
Or even if they have a computer but they don't have the money to replace it when it breaks down , that would mean they couldn't spend money till they buy a new one.
But i do believe a significant portion of the payments will move to crypto.
There is a fundamental piece of the equation that is being missed here. As of right now a Nokia phone costs about $25-50 dollars, but in about 5 years that price will be below $5. Now I know the whole world won't have computers, but nearly everybody will have a cellphone. Once they have the cellphone, and Segwit and LN are activiated worldwide then this whole conversation will be moot. Everybody who is in a "poorer" country now has access to banking (which 80% of this world currently doesn't) so for them its a huge win. For us in the 1st world, does it really matter if we use cash or not? I am what you would call a "Bitcoin freak or maxmalist" but I still don't see the need for getting rid of cash, it is just another form to be anonymous. In fact because it is another form to stay anonymous, I am in favor of it staying for my whole life.
ps wasn't trying to not answer question OP, in the grand scheme of things I think 600k is negligible amount of money considering we will have trillions inside Bitcoin.