Ironically, paper cash is immensely more risky, so not sure why you even brought it up. That cash burns up, you're effed. If you take it across a border or get pulled over, you are likely also effed. If I memorize my wallet key, I can take it anywhere and it cannot be confiscated, it cannot be seized.
Yeah, and if someone else gets that wallet key, it's gone and you have zero recourse to get it back.
Anandtech has had these debates in the Bitcoin posts before. I think that it boils down to this... if you were going to have a crazy weekend in Vegas, which would you rather have: $10,000 cash, or $10,000 in Bitcoin? The answer is pretty obvious, most people going to take the cash because most businesses still don't take Bitcoin without requiring a cash conversion first.
Those few people who said that they want to get the hell of Vegas and "HODL" the Bitcoin also show the other side of the problem... The people who want Bitcoin aren't actually spending it. Bitcoin was designed to be a currency, but it is really more of a store of wealth now. It kinda failed as a currency... the transaction times are slow, its transaction fees are high, and there are other more efficient crypto alternatives out there now.