That's safe. Convenience depends on how often you need it (and so there's not one standard of convenience you can quantify without being the actual person who's using it.)
> That's safe.
It is quite safe, but still not as safe (World Trade Center, Hurricane Katrina, ...) as your money in an account with insurance. And much more inconvenient.
You can also replace "one" with an arbitrarily-large number, depending on how many locations you want to store the money in.
It's not. But compared to storing large amounts of dollars, pounds, or euros, it is easier.
> Yes, you can set up a system that is very resistant to failure. But my grandma doesn't understand the difference between minimizing a window and deleting her facebook account. I'm not excited about explaining SSS to her.
Your grandma won't need to know about any of that, because she'll probably continue to trust an organisation to look after her money for her.
Just because cryptocurrencies make it feasible for people to not trust other entities with their money doesn't mean that everyone will.
You can read books yourself but it's so much easier to just listen to the vicar reading them out instead.
People learn things that are helpful to them.