I think a very large majority of people will be doing transactions entirely cash-less fairly soon. However, there will always be those 10-20% that need to do things the old fashioned way, off the books, for whatever reason.
Cash also can't be stolen as effortlessly
as electronic funds. Much of our financial data is pathetically insecure at the moment, at least in the US. For example, debit cards and are now saved from massive fraud almost entirely by automated fraud detection systems, which are now simply blocking enormous areas from usage: basically half of the US and any foreign country. (Call your bank to turn it off, and see what I mean. Just don't keep it off for long...)
tl;dr: Cash is here to stay, but may become irrelevant for most.