Paying for stuff, I don't think I've really had a problem at the time. I don't know what the maximum you can withdrawal from an ATM is, but I doubt you can get $20k+. I also can't access my safe deposit box 24/7. My dad got all his banking stuff f***ed up because he bought a boat on a credit card, paid it off a few days later, then the bank said that was "suspicious behavior" because most people don't pay off their CC in full.
My grandpa uses cash for pretty much everything and he'd deposit or withdrawal cash when necessary. One day his account got flagged or suspended for "suspicious withdrawals/deposits" (or something like that) and had to go through a whole rigmarole to get it fixed. In the mean time, his social security or whatever it is old people get, stopped going through because of his account being suspended. Now, he doesn't particularly need that SS money, but it definitely helps and is something he's paid into and deserves.
The banks shouldn't be allowed to lock up your account because you're using your money the way you want to, especially when it's being used legally, small amounts, and no evidence or ties to any illegal activity. Your money is literally in their hands and at their mercy.
I had a credit card turned off a while back. It was always paid in full (automatic payment from my checking account). They wouldn't tell me what happened or why it was turned off. It really f***ed me up because I was in the middle of a trip and that was the card I used when travelling, as well as monthly bills like electricity, gas, cable, Pandora, iTunes, etc. and it was also set up through a bunch of websites so I wouldn't have to re-enter my CC information again (like through Caesars hotel reservation website, Amazon, and others). Now the problem was, other than fixing all over those and changing it to a new CC to be automatically billed, I didn't remember all the things that were being billed on that CC, so a few weeks/months later, I'd get services shut off (like Sirius XM and Pandora, IIRC) because the card couldn't get charged. Thankfully, it was nothing important and I didn't get charged interest for not paying.
Yeah, right now there's a bit of a problem of having to secure the coins in cold storage, but I can imagine this would only be a short term "problem". I'd imagine a secure device, like a smart phone (but secure, and specific to cryptocoins) would be made where you can scan a QR code and send the amount you owe.
Yes, if/when there's a mass migration and adaptation to cryptos, it would totally suck (for most people), but that's mostly because the economy would be absolutely dogshit and everything would be all chaotic. Eventually, I think it will happen, once people realize how much they're getting screwed.
EDIT: Also, many of the problems aren't necessarily for us in the USA, but for people in other countries. Our economy is pretty stable compared to some other countries's economies.