-First, you need money to make money. It is hard to imagine a world where the majority of the citizens will adopt this currency. In the first world we can mine it if we invest a few thousand in hardware and some electricity... most of the world's citizens cannot say the same. What reason would a person in N. Korea have to drop $600 on one bitcoin that requires a computer rather than 30 ounces of silver?
Well, N.Korea is a rather strange example - they do not have 600$ laying around unless perhaps they are a government official. People buy stocks of higher prices than 600$ in the first world, I would not be surprised.
While I can buy one today for $600 lets suppose that in 5 years the price is $10 000. How difficult will it be for the citizen of a third world country to purchase a coin in order to "opt into" the system?
They would buy a smaller amount. Perhaps a uBTC which would then be worth more.
This brings us to security. You know to keep BTC in cold storage, copy your .dat files, use a clean boot of Linux or some other operating system, transfer your files on USB sticks... Do your honestly believe that the average person using a tablet is going to understand even a fraction of this? In a first world country where Windows is the standard and the majority of users could not scan for a virus if their life depended on it (well, they might download some malware to scan for a virus should they do a search...) I know the upcoming generation is more computer literate- but we are talking widespread acceptance in all countries, economic classes and levels of intelligence. I think after the third time somebody buys a $600 coin and has it “drained” by hacking they may give up. Again, why would the average person purchase something with these risks?
We have got tons of kids using things like dogecoin. I think it is without a question that we will have an internet currency. People want fast and easy - have you ever tried to buy an App on a mobile phone? The number of forms to complete is simply not a model that will sustain.