A $100 cellphone isn't a decentralized peer to peer network. You are comparing apples to nose-hairs. Bitcoin will scale in the same way the internet scaled... slowly and then suddenly.
[1]: https://www.reddit.com/r/btc/comments/49osb4/examples_of_rbi...
[2]: https://medium.com/@octskyward/the-resolution-of-the-bitcoin...
On the other hand a global payment system would require a currency which is at the very least stable (and not increasing in value). But a stable value might not be enough to cover the expenses of most of the miners. Mining is already dominated by Chinese miners which happen to have access to the cheapest electricity - which incidentally completely undermines the aspect of decentralization.
In summary it seems that at this point a decentralized payment system just appears to be too expensive (computationally) to be any serious competition for currencies in countries with a well established, trusted payment system. It still might be of use in the developing world.
It works in the other direction too. Lowered value lowers mining competition which LOWERS difficulty. It stabilizes itself automatically.
Like I said, Bitcoin will scale like the internet scaled which is in direct proportion to demand. Blocks aren't often full and if you are in a hurry then you need to pay 10 or 20 cents to prioritize your transaction.
But it's hard to get and hard to use. It's useful for a pseudo-anonymous transfer for value.
Unless I'm doing something illegal, Visa is a better option.
There's always one situation where one currency is more convenient than the others.
Is this attitude a remnant of the religious wars in Europe?
There can be only one?
One interesting thing is there is an optimal fee based on the size of the block of transactions.
If you re read my comment you can see I explicitly said getting money out of those countries, which have been infamous for capital controls.
Also - Magic Beans - accepted at 48 locations in Athens https://coinmap.org/#/map/37.99873292/23.73853683/12
Look up the ideal properties of money and you might understand the big picture better, but it may mean challenging your deeply held beliefs and not everyone is ready for that.
But even without game theoretic arguments involving civil disobedience, Bitcoin is still interesting. It offers cryptographic protections that have never existed before in real fintech applications (e.g. programmable signatures), and radical transparency and configurable trust that can patch the sort of vulnerabilities that led to the 2007 financial crisis.