As long as there are still people without access in electricity, cash will still remain. Actually I don't think we would ever go full digital in paying for transactions because first, not all people can afford to buy a phone (there still are in third world countries) and lastly, the dependecy on the Internet would be extremely high and that is not a good thing.
in my opinion, I can't, because cash is still used for spending payments every day. and many people do not understand or cannot use the internet for a day how do people invest with bitcoin.