I'm not sure if disposable means the same thing to you as to me. Gramma may use a smartcard at the grocery store just as she uses a debit card today. If she can't do that, she can't use bitcoin. The card accepts the pruned merkel tree, sends signed transactions, and displays the balance on the card itself. She can plug it into her grandson's laptop if it doesn't have some wireless interface.
Momentum and fear of the unknown are powerful forces. The trick is to introduce technology so that people don't notice. I installed Ubuntu on my ex-girlfriends computer who uses a Mac at work. Granted, I did all the configs, backup, and security upgrades, but she used Ubuntu just about every day for two weeks and didn't realize it wasn't Windows. I think Android is a better example though. Linux is everywhere, you just don't see it. Bitcoin is rough today, but when people don't see the difference between a credit card and bitcoin is when it's mainstream. Within the year we'll have an app on a smartphone, a dialog window will pop up on screen and ask, "Purchase pack of gum .08 BTC from ABC Shop? [Send] [Cancel]" Will you use very often? Probably not yet. But the tech is already here today.